DO NOTHING!!! I REAPEAT - DO NOTHING!!!
There are a lot of good things that can be said about med school, but the amount of work and lack of free time are not some of them. I would say that you should live your summer as if you were going to jail in August, because that's pretty much what it's like. Every day for the most part is go to class, go to library to study, go home and sleep, repeat. Doing things like traveling or visiting museums or anything like that goes out the window as you are confined to solitary in the bowels of the school with gross anatomy your cruel warden.