If you had a book in which whatever you write in it happens, would would you write in it?
I think I'd try out a number of my dreams, and I'd do it by writing "and he went to bed, and the next morning he woke up in the body of so-and-so for just. one. day." Then, when I returned to being me, I could give the book another spin and try something different if it didn't pan out the way I hoped it would. It's easy to idealize every aspect of a fantasy... but you know, the grass is always greener! Rinse and repeat, and after a couple months of running that gauntlet, I could fashion my own little perfect reality.
A couple of the people I'd like to spend a day as... Tom Cruise (what is scientology, anyway?), my Cat (the dude always seems so complacent), Tommy Lee (20 years ago, on tour), Keith Richards (just to be sure he's actually still human), David Fincher (what in his brain causes him to shoot 99 takes of every camera setup?), Kate Upton (..heh), Neil deGrasse Tyson (every time I watch an episode of Cosmos I feel like my head is going to explode, imagine being the voice of it!), the list goes on and on, man.
Second answer (if I may): "And all of the nuclear weapons on planet Earth turned out to be full of confetti, and all of the evil dictatorships were revealed to be populated by aliens, and their presence being outed to the world was so deeply embarrassing to their kind that they fled deep into the outer regions of our galaxy, never to return again. In their absence, Joe Rogan and Louis CK were elected to run the new government, and their first official act was to rid everyone on our planet of the ability to feel anger or frustration. Then, miraculously, food and water no longer became necessary for the sustainability of human life, and everyone stopped aging. The End."