Great question - perhaps more revealing than the 'who would you like to have dinner with?' one.
Personally, though I should probably say someone like Gandhi, or Isaac Newton, or Elizabeth I, would like to be Peter Pan for the day. I don't know, there's just something about flying without the aid of a man-made machine that really grabs me. A bit of fairy dust, a happy thought, Bob's your uncle, you're flying. And I'd really like to fight pirates too - the proper old-fashioned ones, swashing their buckles and swabbing their decks. And who wouldn't want to be a kid forever? See, you say a day, but a day as Peter Pan would last forever, so I suppose that's kind of cheating.