Ways to prove intelligence and competence aside from grades for college?

I'm a sophomore in high school with about a 3.4 GPA and quite frankely disgusted with myself. I go to an incredibly demanding and eminent school, but that fails to make up for the fact that I am working nowhere near my potential. In no way to sound pretentious, but I know that I am capable of great things and much better grades. For the rest of high school I have promised myself to work significantly harder. My grades in no way portray what I know I can do. What else can I do to broadcast to colleges in two years my full potential and intelligence, aside from working on my grades?