Thanks to everyone for their advice! 
@ucbalumnus: I may have enough depending on how I pick my classes for the next few semesters, but I’d really like to take the time to explore more before I go to college. Thank you for all your help!
@DJCordeiro: Thank you for the suggestions! I wasn’t aware that there were summer math programs so close to us, and I’ll add it and your other points to my list of things to research. 
@wis75: Graduating early is an option I’ve considered, but not the one I’m leaning towards at this point. I’m happy to hear it worked out for your son, and also to hear that being younger isn’t too much of an issue at college. I definitely do enjoy humanities classes (this year it’s AP Lit and APUSH), and am planning to take several more in the next few year. Thanks!
@scmom12: (I figured I should clarify that I’m a girl since it’s hard to tell from my username ) I have been taking Chinese for many years unofficially, and I believe I could take AP Chinese next year if I really tried. I probably won’t, just because there are other subjects I’d rather focus on, but I will try to do it or at least get to Chinese 4 by the time I graduate. I don’t think I’ll be taking a different foreign language, but I will think of it, and thank you for sharing your experience!
As for other subjects, I really enjoy English (took AP Lit and there’s one more CC English class) and I like the history class I’m in now (APUSH). I’m also really hoping to take economics, psychology, US/world politics, and possibly another history class.
I don’t think I’ll be doing a PhD–more likely going into mechanical/electrical engineering or CS–but I will definitely consider your advice in case I change my mind.