volunteer at a hospital. feed the homeless. discover what you want to major in college. learn a language. play an instrument. make a piece of art(painting, sculture). write your story. read all the classics in literature(tuesdays with morrie by Mitch Albom, 1984 by George Orwell, great expectations by Angus Calder, watership down by Richard Adams are a few). be active in a church. play a sport (karate, running, or dancing if you haven't started one). say no to cigarettes or you'll look 50 at 20. learn to drive. review politics because you can vote at 18. stay clean because you can give blood without parents permission at 18. get a job.
If you're doing all this, you should be on top of your game by the time you move out & end of college.