I made a top 10 list:
10) Enjoy high school. Or not. Most people agree college is better...
9) Don't cheat.
8) Anything where you get to talk to people. This includes friday night keggers. Make yourself known... (also see #1)
7) If you're really inclined to - start some kind of research volonteer stuff and learn the lab procedures/how to read scientific literature. Then in college you can get in on a big project alot easier instead of washing dishes.
6) Language - can someone tell me any language useful for a practicing physician in the US besides spanish? I'm waiting for someone to say French, but I don't think its going to happen...
5) AP Classes, then re-take the AP classes in college i.e. Chem I, Cell bio, Calc I, II. WHY? Because its an easy A so you can take some 200/300 level stuff your freshman year as well, and if you skip them (as I did), sometimes you get in over your head in the next level of classes. Doing well early in the pre-med req's also makes for some good LOR's.
4) Sports - if it interests you, opens up coaching/leadership/all kinds of stuff
If you arn't co-ordinated, do cross country (often times if you look, the cross country team has some of the smartest kids on it too... I think it comes from the sobriety that many coaches impose... but enough on that)
3) EMT - might as well get certified early and have less to do in college
2) Don't get burned out in HS, no matter what, it's not worth going crazy over
1) Don't get arrested