What are some good ideas for hanging out with friends in the summer?

I'm [M15] going into my sophomore year of high school. I live in Southeast Pennsylvania near Philadelphia. My summer has been okay, but I've only done a number of things with friends.

I have gone bowling, seen a movie, and had some friends over swimming. Do any of you have more ideas of things I can do with my friends before school starts back up? (Preferably legal)