Cynic answer: A few minor neurochemical tweaks. More useful answer: Were I able to maintain the best parts of being a student on a consistent basis, I would probably be as close to happy as I can imagine being. Excellent conversation, Great books, Hard questions. A quiet, comfortable, independent life; but with good friends and a loved one close by. The occasional nice meal out, good puns, libraries, &c.
In practice, of course, I'm not too good at keeping this up for arbitrary periods without the downsides interfering, and it isn't going to get any easier once I need to enter the real world; but as a purely hypothetical situation, that is probably it.
I have two main problems in my life that I would love to be fixed, the rest I can take care of:
(1) Money (I can take care of this of course, but having a sh!tload right now would help)
(2) Stop my gf from trying to move to NZ with her Mom & sis (or in the same turn just stop her Mom & sis from moving )