What enters the mind of a Buddha when left idle? Suppose a Buddha were confined to a hospital bed or prison cell. One cannot remain within the conditioned states of jhana indefinitely. If one is unconditionally content, even in states of pain, it would seem there is no need to reflect or contemplate anything. Can you cite specific examples from the Buddha's life?
From parinibbana suttha
16: The Gracious One’s Sickness
Then the Gracious One, after living near Ambapālī’s Wood for as long as he liked, addressed venerable Ānanda, saying: “Come Ānanda let us approach the little village of Beluva.”
“Very well, reverend Sir,” venerable Ānanda replied to the Gracious One. Then the Gracious One together with a great Community of monks arrived at the little village of Beluva. There the Gracious One lived near the little village of Beluva.
There the Gracious One addressed the monks, saying: “Go, monks, and undertake the Rains Retreat in the vicinity of Vesālī living like friends, like companions, like comrades, and I will spend the Rains Retreat right here at the little village of Beluva.”
“Very well, reverend Sir,” and those monks, after replying to the Gracious One, undertook the Rains Retreat in the vicinity of Vesālī living like friends, like companions, like comrades. But the Gracious One spent the Rains Retreat right there at the little village of Beluva.
Then while dwelling for the Rains Retreat, a heavy affliction arose for the Gracious One, and continued with strong and death-like feelings. There the Gracious One dwelt mindfully, with full awareness, and without being troubled. Then this occurred to the Gracious One: “It is not suitable that I, without having addressed my attendants, without having given notice to the Community of monks, should attain Final Emancipation. Having energetically dismissed this affliction, I could live on after determining the lifespan.”
Then the Gracious One having energetically dismissed that affliction lived on after determining the lifespan. Then the Gracious One’s affliction abated. Then, the Gracious One, having risen from that sickness, not long after rising, departed from the Sick Room and sat down on the prepared seat in front of that Room.
Then venerable Ānanda approached the Gracious One, and after approaching and worshipping the Gracious One, he sat down on one side. While sitting on one side venerable Ānanda said this to the Gracious One:
“I have seen, reverend Sir, the Gracious One comfortable, I have seen, reverend Sir, the Gracious One bearing up while sick, and my body, reverend Sir, became faint as it were, and although I could not see straight, and things were not clear, it appeared to me, reverend Sir, that the Gracious One was sick, but it was some small comfort that the Gracious One would not attain Final Emancipation until the Gracious One had spoken regarding the Community of monks.”
“But what, Ānanda, does the Community of monks expect of me? The Teaching has been taught by me, Ānanda, without having made a distinction between esoteric and exoteric, for the Realised One there is nothing, Ānanda, of a closed teacher’s fist in regard to the Teaching.
To whoever, Ānanda, this thought occurs: ‘I will lead the Community of monks’ or ‘I am the instructer of the Community of monks’ let him speak, Ānanda, regarding the Community of monks. But to the Realised One, Ānanda, this thought does not occur: ‘I will lead the Community of monks’ or ‘I am the instructor of the Community of monks’. Then why, Ānanda, should the Realised One speak regarding the Community of monks?
I, Ānanda, at present, am old, elderly, of great age, far gone, advanced in years, I am eighty years old. It is like, Ānanda, an old cart, which only keeps going when shored up with bamboo, just so, Ānanda, I think the Realised One’s body only keeps going when shored up with bamboo.
When the Realised One doesn’t pay attention, Ānanda, to any of the signs, when all feelings have ceased, he lives having established the signless mind-concentration, and at that time, Ānanda, the Realised One’s body is most comfortable.
Therefore, Ānanda, live with yourself as an island, yourself as a refuge, with no other refuge, with the Teaching as an island, the Teaching as a refuge, with no other refuge. And how, Ānanda, does a monk live with himself as an island, himself as a refuge, with no other refuge, with the Teaching as an island, the Teaching as a refuge, with no other refuge?
Here, Ānanda, a monk dwells contemplating the nature of the body in the body, ardent, fully aware, and mindful, after removing avarice and sorrow regarding the world; he dwells contemplating the nature of feelings in feelings, ardent, fully aware, and mindful, after removing avarice and sorrow regarding the world; he dwells contemplating the nature of the mind in the mind, ardent, fully aware, and mindful, after removing avarice and sorrow regarding the world; he dwells contemplating the nature of things in various things, ardent, fully aware, and mindful, after removing avarice and sorrow regarding the world.
Thus, Ānanda, a monk lives with himself as an island, himself as a refuge, with no other refuge, with the Teaching as an island, the Teaching as a refuge, with no other refuge. For whoever, Ānanda, whether at present or after my passing, lives with himself as an island, himself as a refuge, with no other refuge, with the Teaching as an island, the Teaching as a refuge, with no other refuge, those monks of mine, Ānanda, will go from darkness to the highest—whoever likes the training.”
The Second Chapter for Recital is Finished.
From the Ayacana Sutta (SN 6.1):
Then, while he (Buddha) was alone and in seclusion, this line of thinking arose in his awareness: "This Dhamma that I have attained is deep, hard to see, hard to realize, peaceful, refined, beyond the scope of conjecture, subtle, to-be-experienced by the wise. But this generation delights in attachment, is excited by attachment, enjoys attachment. For a generation delighting in attachment, excited by attachment, enjoying attachment, this/that conditionality and dependent co-arising are hard to see. This state, too, is hard to see: the resolution of all fabrications, the relinquishment of all acquisitions, the ending of craving; dispassion; cessation; Unbinding. And if I were to teach the Dhamma and if others would not understand me, that would be tiresome for me, troublesome for me."
As the Blessed One reflected thus, his mind inclined to dwelling at ease, not to teaching the Dhamma.
Then Brahma Sahampati, having known with his own awareness the line of thinking in the Blessed One's awareness, thought: "The world is lost! The world is destroyed! The mind of the Tathagata, the Arahant, the Rightly Self-awakened One inclines to dwelling at ease, not to teaching the Dhamma!" Then, just as a strong man might extend his flexed arm or flex his extended arm, Brahma Sahampati disappeared from the Brahma-world and reappeared in front of the Blessed One. Arranging his upper robe over one shoulder, he knelt down with his right knee on the ground, saluted the Blessed One with his hands before his heart, and said to him: "Lord, let the Blessed One teach the Dhamma! Let the One-Well-Gone teach the Dhamma! There are beings with little dust in their eyes who are falling away because they do not hear the Dhamma. There will be those who will understand the Dhamma."
From the Tapokammasutta (SN 4.1):
On one occasion the Blessed One was dwelling at Uruvela on the bank of the river Nerañjara at the foot of the Goatherd’s Banyan Tree just after he had become fully enlightened. Then, while the Blessed One was alone in seclusion, a reflection arose in his mind thus: “I am indeed freed from that gruelling asceticism! It is good indeed that I am freed from that useless gruelling asceticism! It is good that, steady and mindful, I have attained enlightenment!”
Then Mara the Evil One, having known with his own mind the reflection in the Blessed One’s mind, approached the Blessed One and addressed him in verse:
“Having deviated from the austere practice
By which men purify themselves,
Being impure, you think you’re pure:
You have missed the path to purity.”
Then the Blessed One, having understood, “This is Mara the Evil One,” replied to him in verses:
“Having known as useless any austerity
Aimed at the immortal state,
That all such penances are futile
Like oars and rudder on dry land,
By developing the path to enlightenment—
Virtue, concentration, and wisdom—
I have attained supreme purity:
You’re defeated, End-maker!”
Then Mara the Evil One, realizing, “The Blessed One knows me, the Fortunate One knows me,” sad and disappointed, disappeared right there.
Buddha did Arahata Phala Samapatta (Extinguished, emptiness and neglecting any sankhara needed to be maintained, in brief Buddha was in the state of leaning Nirvana), in between his daily routines and his discourses on Dhamma.
When people are in aversion each other Buddha stayed with Metta, when people are in vain with debate/disagreement Buddha stayed with Upekkha. Buddha looked for people with sufferings to discourse Dhamma with Karuna Samapatta in every morning.