Mr./ Mrs. Smith, that is a very thoughtful question. Generally you may offer what ever you feel inspired or feel that it is of use, regardless if the Monk might be allowed or willing to receive, would deed in intention is done.
Some give a flower, just picked for their joy, some a shady seat to rest, some a joyful and devoted bow or folded hands, some a wish "May the Ven. gets all he need for his way. May he never miss anything."
My person assumes here, that Mr/Mrs Smith asks specific of what putting a alms-food searching monk in the bowl.
It's maybe good to count here some things he might not be able to receive or make use of it:
raw meat, raw fish
certain kinds of meat, like snakes, tiger, bear, dog, horse, human
uncooked rice, corn, seeds, flour... (food that need to be cooked fist), some receive instand food
he might have difficulties with whole fruits, coconuts, if not having lay assistence... since not harming sheeds.
Aside of that, there is not much, he might not able to take.
It's how ever good to regard a monk always like a small child in regard of food. Meaning to maybe even think how he can it, what he might need... if having possibilities, but at least it's better to give the half of you hot-dog you just eat, or the candy you just wanted to take, then to deicide you have no luck to be not able to give what is proper to give. Something people often cut of to use a seldom oppotunity. What ever you might have at home, maybe a half apple from yesterday, is great and it's always higher merits to give that what one has, or hangs on, than to organice or take for such a sake.
It's also not a problem to give others then food, if inspired, if seeing an alms-begging monk, like maybe wash items or medicine. A lone living Tudong-monk depends nearly in all exclusively on his alms round and wise people. Sometimes even a Dhamma-book is a good gift.
It's of course good and a sign of obligation, to thinks also further: to give proper amount and what he might be able to use and carry. How to open a conserve for example. There are those not using dishes, spoon. It could be that his bowl is full of rice and not other food would have any more place.
It's also totally not wrong to ask to take a look into the bowl, so that one could see what is missing. It's also not wrong to ask for the bowl to fill it. It's also not wrong to ask for waiting, offer a place to sit, and prepaer food, or spontanious invite into the house or a place to receive food. In such cases, when inviting to take his meal, it's good to think that he will maybe not provided with later requirements, like drinks and tonic. Speaking as exclusively alms-goer with no other support, it's good to think also on water and tonic-drink for the afternoon. For monks living in that way, what ever is given on his round, that is his nourishment for another day, no store or another source. Some eat also just one time, exclusively what is given on the alms round into the bowl.
It's not bad to think in that way always, even if in the city. One does not know normaly a monks circumstances. So it's not good to think in ways like "oh, that he will provided there and there" or "that will be organised in his monastery".
Best thinking like if meeting a child that would not claim.
Hope that gives a more lighter and joyful spectrum of the many chances. And Mr/Mrs. can not make anything wrong, that's most importand! Just being "creative" and circumspect without giving selfish thoughts and doubts any change in such blessed moment, is the best.
If visiting a monastery, best to let you be assisted by lay assistance there. Importand: since you may give also things to eat for later: What ever a Monk has received on food, he and the other Bhikkhus would not be allowed to eat next day, even if again given. So such food exclusively giving to lay people.
When giving food direct, respectfull and with two hands into the hand of the monk and don't touch, or lift it again.
Give food or what ever, before receiving Dhamma, e.g. as fist thing best, when coming to monks. They might not accept after having given themselves.
It's a good topic and there are often many wrong assumings "a monk eats this", "needs that...", so good to expand if questions arise.
Now my person, nevertheless needs to prepare for his alms-round, so he stopps here.
A warning: Be careful of that what ruins a fool who having gained knowledge! He/she might think in ways like: "The monks must... His duty is... so and so he should receive..." Having gained much knowledge about possible duties of others, not really know practice and as it really is, and forgetting on focussing on wn mindstate, which is the A&O of all here, one makes nothing then demerits, distroys his goodness, with every act one believes to make merits at least. Generosity that is path-supportive requires not only wisdom but also right view. A normal person or even a fool is not able to go or came as even far as this.
May person has to declare a praise here:
Mr/Mrs. Smith question, deed, is an outstanding. Such is the way of a person with upanissaya in the tradition of the Noble Ones, and one with such an attitude can nothing but prosper on the path. Entering a place expecting to meet admirable friends, he/she did not ask for anything but: "Friends, how can I give? What can I give? How can I fill needs? How to serve our teacher, those elder, those guide us, first?" Everybody taking this deed as sample, train his attitude in this way, does not ask and demant at first place, does not speak about rights, but looks of how he best can give and serve is naturally a winner on this path, a person of great wealth a person who will respected and venerated by those of good conduct and wise, where ever appearing, what ever assambling he/she enters, will never have the idea of being poor and in need.
(My person will try to invest this day, to complet the answer and give as much as possible hints for not getting caught by Mara-kileasas, add talks and readings to have most accessable base for developing the base)
Possible importand Additions:
Don't break precepts, harm your virtue for the ake of making Dana:
What ever is near and own is good. Do not take, even kill, ly... for the sake of giving. Even is good made, the taking and giving, it's maximum same, but does not bring benefit at least. One who has really nothing to give is gifted, since he can focus one more refine and greater gifts, that of virtue. Sometimes to let go of desire for making Dana can be of more merits than such outwardly act. But beware of self cheating and excuses here as well. Defilements are smart and quick.
Beware of mara-kilesas, demons of defilements:
There is a nice talk with good story by Ven. Ajahn Lee, that gives good explainings: The Demons of Defilement: (Kilesa Mara)
Do not give to win or harm:
There are many people who give to win, gain in reward wealth, honor, influence, favor. Such is of short wordly merits althought it might work as well. There are even many people who give the highest gift of Dhamma with a thought of harming others with it, and to fight something. That is why the Buddha bound "not harming others with your gift" on what makes a benefical gift.
Juice as tonic for the afternoon: There are some fruits allowed by the Buddha, where juice can be made from, such as citrus fruits, appel, mango... (general tree-fruits and no big fruits). If wishing to serve such, the juice nedd to be cooked then filtered and before serving made cold. Such couldbe keep one day and taken in the afternoon. Sugar might be added. In regard of packed juice, not all monks and communities are not afraid to receive it. It's sometimes not easy to find out if the juice is made in such a way and what it contains. Coffee without milk and tea can be served.
This topic migh be handled most different by certain communities. No problem to ask in advanced "Are the Ven. Sirs able to receive and make use of this or that, if given?" Note that silence usually means not "no", but let you freedom to take action as wished.
Main dishes - Rice nessesary?
No, not at all. Bread, noodles, potatos... what ever main nutrishion, if wishing to give a whole dish, is fine. It's maybe worthy to tell here, that it is not usual to receive/eat more side food ("curry") that 1:4 in regard of "rice". So a monk would possible have not enough food is given a western style dishes with much main food "side-food" and less side-dishes "main food".
Sources for more about food and medicine:
One who knows much about others ways, can help much. Used in this way is of benifit for the user. Used in a way to revile or blame others, even if right, but with a unskillful mind, is of no benefit for one. As giver you are not obligated to know what is needed and proper to give, but if liking to be a perfect giver and have joy with it, it's of good use:
BMC 2, CHAPTER 4: Food
[BMC 2, CHAPTER 5: Medicine (incl. Tonics for the after noon and food for sick monks)
Putting the food in the bowl
Since not proper and not usual to lift the cover of the bowl wide, but food possible careful into it, one by one. It's good not to touch the bowl, it might harm the surface and it's not easy to fix such for a monk (in Asia, out of this, the folk believe that touching the bowl brings bad fortune has been developed). If you wish to give the side dishes, thinking on possible carring about the monks wefare, in small begs (think on that he has possible no way to care good about packing material, is here also good) or put the side dishes as they are into the bowl. Good to consider that some monks go not only for them alone for alms, but share the food under fellows. Some might have themselves or assistant with them, to receive side dishes, sweets, fruits... in kind of small "picknick boxes". Follow as it looks proper for you and the situation and as you think it's good. Some monks may have not a slight problem to take how ever it comes, some might not have the freedom yet. Don't fall into ideas like: "He should, must...", that reduces just you own benefit."
Frozen, if not ice-cream, is maybe not so easy to take, but well, if ready to eat. Temperature does only play in regard of what you like to give and possible for receiving play a rule and there will be no requirements at all. To hot: is a monk, althought it might be seldom to meet, carries his bowl with his hands, not on support, no protections, the heat you but into the bowl will touch fast his hand as well, can hurt and make him even to let fall his bowl or not able to carry. One might be cateful on such in regard of fresh cooked rice. In a case such happens, one maybe likes to offer some leaves or a piece of garment, or what ever idea one has.
Recources for Dana and its practice:
Recommended Dhamma-talks and Essays:
Outwardly Foundation for Practice - Admirable Friends:
A person who wishes to prosper to and on the path should avoid to associate with greedy and stingy people, avoid places where demanting, claims and rights are praised and assossiate with people having joy in giving. Not doing so, less joy and learning can be expected.
Inviting people to take part on ones deeds, direct or indirect is very conductive and additional merit. Helping in doing Dana might gain even more merits (because of ones joy and mind state) than the actual giver might gain.
The best wish you may give as crown on your alms, as far as merits can benefit in the world and beyoun (from my persons view, Note it's not supra mundane yet)
May the Ven. accept this offering food of mine, and may this be a support for the highest and most compassionate goal he strives and has left home, to become another Arahat soon if not yet, have long live, honor, well-being and strengh, so that he might be an unexcelled field of merits for many a long time.
In the case he fails to enter the path or receives it without such aspiration and effort, you have one in dept and a future servant, slave or supporter. In the case he matches the high value of your gift, you might never fail to have best connection to the Noble Ones and the path. (This is how big and good rulers, leaders, with lot of servants and surounded by wise appear in this world)
Of course such a wish has to be honest and from deep in the heart, and not just a win-win strategy. Pure heart always wins, there is no lose in giving at all.
On formal citations (note: not required), see: Formal Offerings
Doing in three ways
While "nobody" can judge your mind, it's most importand. As for being aside of that acceptable and benefical, take on a proper body gesture, like step out of you shoes, give not from below but as a request to be accepted, act humble, careful and not fast... speak out what you wish to do and your dedication with humble words and understandable, so that there is no doubt of what you do outwardly. Just giving could be led or what ever. You might wish that he/she holds it for you later.
In you deed while preparing, while doing, and afterwards, always remember it, and wish to be able to do it again. NEVER think in ways, what ever reason, "it might have been better to don't give..." and brun your field of fruits of merits with such.
(Oh, yes, don't forget to share your merits, invite others to rejoice with it/you and tell about you good deeds, not to higher yourself, but as inspiration and encouragement for others, for the joy of those able to have joy seeing, hearing and doing about good deeds. Invite people near and far, on the street, others next, to help with and join you giving.)
A tip in reagard of organised monasteries:
Modern or western tendence is make things productive and efficient. That can of course led whole communities astray and if eager people manage places, it can be a joy-killer and develope things like "If you are interested in offering a meal, please submit a donation form or contact the monastery". Aside of the good hint to seek for best empathy and good-will, like understandig, one should not assume that management, monks, single monk is one. Sometimes it's good to seek direct ways to a monk and go around caretakers and managers, who of course might have best intentions. Small places, humble places, places who do not put demants at first place but openly receive and let people best do their merits and can take imperfectness are often more joy and of more fruits. That does again not mean that it is wise to be not very responsible with your freedom for you benefit.
(Note: This is a gift of Dhamma, not meant to use for commercial purposes or other wordily gains, but intent for liberation: so to share without bounds to anything else)