One thing I'd do is write out a scarily wordy letter, for a 5 year old, explaining to my dad that he should invest in Microsoft and Apple ASAP, and Google, as soon as it exists.
I'd make numerous changes to myself, but actually live basically the same exact life, because gods forbid I change any of the people most important to me, and they not end up who they are today.
I'd possibly get with one of my closest friends and serve as her moral compass and emotional anchor as best I could before she could drift off into the dark times of her life. And hopefully I'd leave her much better off than she was in reality for the last 8 years or so.
(is that.....wrong? by that point I'd have a....37 year old mind, but she and I would technically be like...16... Whatever. Weird situation, so it sounds good to me.)
Eventually I'd, again, get with ANOTHER one of my closest friends when she was going after me back when we first met, and hopefully allow her to shortcut around a whooooole lot of horrible crap, including what's going on currently.
(at least she'd be like...*sigh*...18 by then. Damn her for being 2 years younger than me and not at least being 21 by this point.... my mind would be like...41. You know what? Bigger age gaps have happened in regular people, so...whatever. I'd be doing awesome for a 41 year old XD)
And hopefully, despite my meddling and relationships with her best friend and a girl she knows, a third girl will still end up being almost EXACTLY who she is now, and I'll do what I can to apply the relatively infinite wisdom and insight gained from another lifetime to try and end up with her.
Hell, mentally 41 year-old me might actually be mature enough to properly keep up with 23 year-old her! Maybe not...
I'd do fantastic in school with basically no effort, and boggle minds.
Happily, I could EASILY fake being a kid.
Saturday morning cartoons? Awesome.
Video games? Awesome.
Action figures? Awesome.
I'd start building "old school" collections of games, action figures, comics, etc while they were brand new, amd I'd keep them in super good condition for the glorious future!
I think I'd end up being the reason my friends became aware of 90% of the awesome stuff they ever encountered XD
I'd always know ahead of time which games were badass and that my friends should all play....>_>
I could freak people out by predicting what happens on LOST!!!
I'd have to suffer through the shitty version of the Internet....eeewwwwwww.....
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What a crazy train of thought... You have to figure out what would change, and what you would purposefully change, because you can't POSSIBLY actually change NOTHING if you've already experienced 21 years into the future and are an adult masquerading as a normal kid, so you may as well try and set things up for the best.
Well, this has been REALLY REALLY GORRAM WEIRD.
Entertaining though.
A 21 year mind-body discrepancy is a very very strange thing to contemplate.