Train. You're like a professional athlete during the off-season. You get a lot more flexibility with your schedule and a lot fewer people watching you as you work, but you've got to spend this time productively. Does this mean to fill up your day with summer school classes and be a constant student? No. Like pro athletes, find another way to enhance the skills you have.
Think about your notable skills and talents, the things that you may want to emphasize on college applications. Now find interesting ways to hone those skills and talents. How can you best prove and improve your resilience, passion, intellectual curiosity, initiative, talent, creativity, empathy, or leadership? The more unlike another high school class or program the activity is, the better.
So, for example, going to a weeklong camp for debaters is good, but volunteering to do door-to-door canvasing for a local political campaign is better. Reading books ahead of time for your 11th grade English class is good, but starting a book club that focuses on foreign or obscure books is better. Taking a class for adults at the local community college is good, but teaching younger kids in a summer program is better. The most important thing is that you focus on yourself and the qualities you want to improve, not focus on a vague sense of "looks good to colleges." Do everything you can with your summer time--in any setting, be it a summer job, summer camp, traveling, or staying close to home--to be a become a better person, not have a better résumé.
Push yourself. If you need to work or want a job, that's great. As far as college is concerned it really doesn't matter what that job is so long as you work hard at it and are reflective about what you learn from the job. As you go to work, remind yourself to work as hard as you can. And when you're done, ask yourself what you learned from that day's work. Those two things matter so much more to everyone than the job title or name of the company.
If you don't need to work, then make other plans. And here's the trick: treat it like a job, in the sense that you decide to do your best and be reflective. Even if you have the cushiest summer imaginable--maybe you're going to spend two months as a VIP on a cruise ship sailing around the Caribbean--you can still get a lot out of this. Just begin each day reminding yourself to make the most of the day, and end each day reflecting about what you learned. Whatever is you do, it can be useful for your college applications and useful for your productive and interesting life.
Go someplace new. Choose someplace you've never been that you can visit this summer. Geographically, it doesn't matter how close or far the place is, so long as it's new to you. It can be another country, another state, or another neighborhood. Try to get a sense of how people unlike yourself spend their days, and do it with an open and empathetic mind.
Goal of 20. Another way you can make the most of your summer is to give yourself a goal of 20. Make 20 visits to local museums or parks. Have 20 intentional interactions with older members of your family asking them about their experiences. Read 20 books. Watch 20 of the best movies of all time. Find 20 items to donate to charity. Run 20 miles, spread out over as many days as you need. The number 20 is arbitrary, but an arbitrary number helps make a vague idea an achievable goal. Every time you knock out one of your 20, remind yourself to be deliberate and reflective.