It really matters not what you do, provided you do something beyond simply "hanging out" or only
working on college applications. First, take some true vacation -- that's what the summer is for and you need to rest your mind after finals, etc. Even mids get time off.
That said, the average h.s. summer is 10-12 weeks long. So, even if you take off a full month, that still gives you plenty of time to do other things. Some people need to work to pay for college or otherwise; others want to work. Some people do sports or science camps; others may not be able to afford them or not have any near where they live. Some people volunteer. Some attend summer school to get ahead or take courses not offered at their h.s.
I've had candidates tell me they don't have transportation so that's why they did nothing. Not a good excuse. Volunteer in your neighborhood. Cut lawns. Make meals for elderly or disabled people who live near you or offer to clean up their yard/home. Become a dog walker during the summer. "Babysit" your neighbors' kids. Use the Internet to do something productive. Do a lot of reading. Learn a new skill or sport. There is a world of possibility out there to do something . . . anything . . . other than sleep and play video games for three months. It need not be "impressive" because some folks can't "afford" (in one way or another) to do that -- and BGOs and USNA understand this. Rather, it should be something you enjoy and/or something that helps you grow -- athletically, mentally, socially, etc.
As stated in the sticky above, people who are most successful at SAs are those who like to keep their minds and body active. So be one of those people!