What should I do the summer after freshman year?

Go work at a summer camp. Now, you have to be someone who enjoys physical activity, being around kids, and is willing to be exhausted at the end of day. If so, it's an awesome experience. I did it, and it's worth it.

1. You get paid to drive a boat, send kids down water slides, play sports, etc. For me, that sounds like a decent way to spend a summer. It's low stress, and you probably won't ever get a chance to do something like it again.
2. You'll go back to school with a sweet tan and in better shape. Yeah, it sounds dumb, but it's hard not to get tan when you're on a lake, and being outside all day helps get you in shape. Plus, most guys will probably work out in the morning.
3. A lot of attractive people work at summer camps. This needs very little explanation beyond the fact that you are with a lot of athletic, college aged guys and girls all summer. Also, beyond physical attractiveness, everyone is normally laid back and great to get to know. It's always nice to be able to go to another school or another city and know someone who's couch you can crash on.
4. You get a lot of life experience. Have you ever tried being in charge of middle school boys for a week? You learn a lot about yourself. You have to find ways to take charge, motivate, adapt, and not lose your mind. You'll get pushed outside your comfort zone, which is guaranteed to make you grow as a person.
5. It's an interesting story to tell in interviews. "It says here you worked as a camp counselor last summer?" "Yeah, I decided rather than travel or get an internship where I wouldn't learn much, I would take the chance to serve others. I was able to teach kids how to sail, play a sport, etc. and to be a positive influence in their lives for a week. I had a great experience while I was able to grow my leadership abilities and meet a lot of great people from different walks of life."

It's not for everyone, but I would encourage anyone to work at summer camp after their freshman year.