Summers of lounging about and wasting time are far behind you now. Universities want to see time well spent in the summer. This could mean traveling, working, studying, volunteering, doing research- anything that can help you better understand who you are and what you want to do with your time and your life.
I did a combination of these things- working in an office the summer before my sophomore year, doing a summer program before junior year and shadowing a doctor before senior year. I have seen that summer programs in which students take classes or learn other skills are beneficial. Most colleges offer some sort of program for high school students- take advantage of these options! Once again, go further into your interests.
Do you think you like economics? Take a course over the summer or try to contact someone in this field and see if they can set you up with an internship. Tap into internships offered through your high school, talk to family friends- there are many ways to learn about possible career paths and colleges like when focus begins early.
By putting yourself into a professional environment you are showing the university that you are ready for the next step. Who knows, you might even get a letter of recommendation out of it!
From personal experience, I was lucky to shadow a doctor last summer and solidified my belief that I want to go into medicine. This gave greater focus to my studies, application and overall college process. This is not to say if you are unsure about what you want to do that have no hope. Any experience in any field is valuable and will help you grow and better understand your future and the world around you.