Relax. You've made it very far. And you still have a long way to go. Medical school is like running a marathon. One of the most important things you can do is to prepare your mind by letting it rest.
Try not to study material that you will be learning your first year of medical school. The volume of information you will be presented with during your first year will be tremendous. Without having previously experienced receiving this amount of information, it's hard to prepare by studying. What you go over in the couple of months before med school will be a drop in the bucket compared to what you will be responsible for later. And you will have lost your summer.
Consider taking a short pre-med course if you are seriously worried about being able to handle medical school. Many medical schools offer 4 to 6 week programs during the summer that introduce entering medical students to the rigors of a medical school curriculum. In addition to getting exposure to material, you will meet some of your classmates. And the grades you get don't count. Sometimes you have to pay for these programs, sometimes they're free, and sometimes you even receive a stipend for participating.
Do something fun! Go on a vacation to the Bahamas. Take a cruise down the Mexican Riviera. Enjoy this time as much as possible because once medical school starts, you will have much less free time.
Learn about a new topic. Are you interested in economics? Read Economist articles online. Watch free online lectures from Yale or MIT. Venture into some other field that you haven't learned much about. This will be important to maintain once you are in medical school. You will need to have outside interests.
Set up an exercise routine. Now is the best time to start working out. You could concentrate on strength training, cardio, or a combination of the two. If you make it a habit now, then it will be easier to continue once your free time gets crunched during medical school.
Learn a new skill. Maybe you can't cook. Or dance. Or play pool. Or do any number of things. Find something you've wanted to do for a while, and work at it.