About to finish college so take this from me, i was in your position not so long ago:
Dont try to get ahead of the class in college by learning A level content during the summer, its pointless, just dont do it.
Do try to volunteer in as many different fields as you can, not only does this help you chose what you will want to study for uni (most people end up deciding just one year from where you are now), this also looks great on university, job applications. It shows a degree of thoughtfulness as you show you've considered other opportunities.
Do have fun, you wont be having much of that during college, education only gets harder D: (Some say its easier in uni, idk)
Learn something new, a language a hobby, a sport, anything.
Take a job, if you've never had one yet, its more than the money, its about the experience you gain from dealing with a whole variety of people and their temperaments, and the responsibility.