I had an idea for The Last Calvin & Hobbes Story. I have seen some pessmistic C&H finale cartoons created by third parties, but this is my optimistic one.
Basically, Calvin is all grown up but he has lost Hobbes years and years ago. He comes back to his old house to look for him. Then, boom! Young Calvin and Hobbes explode out of thin air piloting the cardboard box time machine. They chat for a while. It transpires that young Calvin is about to turn seven.
Young Calvin decides that old Calvin needs Hobbes more than he does. Hobbes stays with old Calvin, young Calvin goes back to his own time. The end.
The critical part is the ambiguity. Did Calvin really travel through time? Or did he just lose Hobbes and then find him again after years and years? The critical part is that it doesn't matter.