Anything! You can do just about anything, I mean it. In terms of preparing for the next three years of high school, preparing for college, and preparing for productive adulthood, there's no magical activity that you really must do to get ready. Do your thing, no matter what it is (within reason--if your current thing is chaotic or self-destructive then take care of that first).
At this point, what you do isn't nearly as important as how you do it. Whether you're doing amazing, once-in-a-lifetime things like volunteering with veterinarians at a wildlife refuge in Botswana or mundane things like babysitting your little brother, you can make the most of it. Be reflective. Ask yourself "how did today go, and what can I do tomorrow that will be interesting?" Every day. Read something that relates to what you're doing. Even if all you're doing is walking aimlessly around the neighborhood trying to find someone to hang out with, stop at the local library and learn about the history of your neighborhood and go inside shops you've never been in before. Take photos of weird things you notice around the neighborhood. Be engaged with your world and your mind, whatever you're doing in the world.
Write about your experiences. Writing about what you do on a regular basis serves several goals. For one, any college-bound person has got to be very comfortable with a lot of writing, so practicing on your own with your own assignments helps build up your discipline in a way that's more palatable to you. It will also help you maintain and deepen your self-reflection that's so vital for your off time. If nothing else, journal-keeping now will give you plenty of material to laugh at yourself later. I know that may not seem like a good thing, but nothing helps you feel more grown-up later than being able to laugh at your younger self.
Make a product. Toward the end of the summer, make some sort of product. Select some journal entries to make into full-on essays. Select and edit some photos to make a narrative photo essay. Make an interactive map of the places you visited. Make a book of advice for someone starting the job that you worked. Again, you can do almost anything. The idea is to curate and edit your experience into something that you can share. That's kind of a definition of education, isn't it?
Meet someone new. Right, of course you're going to meet new people over the summer. But what I'm talking about is to pro-actively and intentionally introduce yourself to new people that you've chosen to meet. Circumstance, coincidence, serendipity, and providence bring all sorts of people into our lives. That doesn't mean we can't or shouldn't work to bring others into our lives.
Practice some form of meditation and contemplation. There's probably no better gift you can give yourself than to start the habit of meditation and contemplation. There are dozens of different traditions and techniques to fit any religious, cultural, and personal background. Here is a pretty solid introduction to 23 of them. Choose one and try it. It doesn't have to be a religious or spiritual exercise. It can just be good relaxation.