Go travel. Doesn't have to be big expensive trips, but go do something fun. It's the last summer before college. Friends will start staying at school for summers, getting internships where they go to school, all that fun stuff, so this is the last chance to be with them. I'm glad I decided not to work my last summer, and instead did a bunch of camping trips. You can do Yosemite for a week for super cheap, just make sure to reserve a spot now. We got lucky and did walk in at camp 4 I want to say. National forests are super fun as well. Just go explore stuff.
Wiley Miller a map near you, chuck a dart, find what national forest it hit thats close by, and go spend a few days there. Do this over and over.