Assuming you have the skills and aptitude for college level work, another possibility is to sign up for a class at a local college over the summer.
When it comes time for college admissions, one of the main things that admissions officers look for is the ability to excel at college. Few things demonstrate that better than, well, excelling at college courses.
One thing to consider is that certain graduate professional programs (Law school, Med school, etc.,) consider all college course work, even college courses taken while in high school, when calculating your GPA for admissions.