If you can't touch type, take a typing class. It's easy and a very valuable skill that'll serve you well for many years.
Hmmm, if you're thinking about engineering or science, a shop class would probably be a good thing to take. Hands on experience is something that's underrated until you need it. It'll help you in the few college classes where you have to fabricate stuff, and overall it'll help you conceptualize things better. I didn't take shop but kind of wish I had. I had basic hands on experience with tools and woodworking at home with my dad, but I think I could have learned things from shop.
Also, if you're going for a technical major in college, take as much math and science as you can take at the high school level. The math will bitch slap you in college if you're not ready for it.
johnjohn320 is right, build up the extracurriculars early on, you can't just wait and join organizations in your senior year and attain leadership positions, when you've suddenly figured out you need them for scholarship applications.
Get some kind of cushy on-campus job in college.
Read books, stay away from teh internet
EDIT: Crap, this is a biggie. If you're not physically fit, GET fit. (If you're OK in that department, ignore the following).
You have the internet and access to all the fitness knowledge you could want. If you're a little or a lot overweight, it will NEVER be easier to get slim than right now. Whether you're overweight or not, lift weights now and put on muscle. It will NEVER be easier and it's something that will literally help you the rest of your life. Even if you let that muscle atrophy over time, you can get it back quicker than if you had never built it in the first place. I'm not kidding about this, if I could turn back the clock, this would be #1 on my list of priorities.