To be clear, this is not the first revision of this idea, but its more or less the best sketch in terms of how the whole situation plays out.
Sketch
Old man - 72ish on his death bed. Makes a bet with the Devil. The rest of his days for a single day of his youth to re-live.
Chooses the day his 1st true love rejected him; late in his senior year of high school. He plans to change it, the bet is that whatever he does today will not change the outcome of his life.
At 1st he is thrilled. He is young and fit, and savors simple things like healthy kidneys.
Sees his mother and his childhood best friend, both of whom he has not seen alive in decades. At the Diner, best friend is making purposely dumb jokes; funny. Importantly, his friend tells the titular joke:
"Why did the monkey fall out of the tree? Go, on ask me.... go on!"
"okay, why did the monkey fall out of the tree."
"He was dead."
He tells off the vice-principal.
He sees _The Girl_ and proceeds to fall in love all over again. With a lifetime of experience and confidence, things look like they might go better; at first. But he is overwhelmed by the combined weight of his past memories and a lifetime of his fantasies about what "should" have been. He forgets his age and what that means. The result, ultimately, is he's rejected again. Despite some superficial changes, The Girl, had made up her mind long ago about him, such that the time, place, and circumstances are basically the same as the first go-round.
He realizes that he now has not only lost The Girl, but also lost his soul to the Devil. He is very distraught. So much so that he makes a last desperate attempt to prove the Devil wrong by driving onto the railroad track.
The sound of train whistle turns into the sound of the Devil's laughter as the clock on the car dashboard reaches midnight. The light from the train becomes the light above the Old Man's death bed. The Devil's laughing fades away to the sound of Grandson weeping at the foot of the bed.
It's better without this next part, but if they make a movie, audiences will want a happy ending, so--
"What's wrong, kid?"
"Grandpa, the Doctor says you are not going to make it."
"He's right."
They have some sort of conversation that makes the Grandson change his mind about something established in passing before the Old Man made the bet. The Devil (who is disguised as a doctor in the hospital) drops his clipboard, to symbolize that the old man beat the bet somehow and can keep his soul.
Conversation finishes like this:
"I'm getting sleepy now."
"I'll get the Doctor"
"No, don't. I'm ready for the big nap."
Grandson starts to cry.
"A good friend used to cheer me up when I felt sad, Malachi. Malachi, come here. I want to tell you a.. something."
Realizing that he alone will hear his grandfather's last words, Malachi leans forward attentively. The Old Man starts to fade. Gently, Malachi rouses the Old Man, who finally says:
"Why did the monkey fall out of the tree?"
