No need to freak out yet! For most people, it wasn't that much of a huge transition from college life. I think it's normal for people to start out a little on the intense side, and then relax more and more as the year progresses. For fun stuff -- a lot of the stuff that I did prior to med school, and I think that went for most of my friends as well. Some of it will have to change because of location, but you will still have time for movies, dinners out, the gym, stuff like that. A lot of people in my class started new hobbies as a way to channel nervous energy and to get to know their classmates better -- stuff like dance classes, the rock climbing gym, golf, etc. Relax and good luck.
I agree with many of the other posters. For me, it is imperative that I take time to do the things I did before med school started. I have a great set of friends who will be critical to keeping me grounded the next 4 years. I enjoy going to the gym and think that the stress relief it brings will be welcome. As with college, there are activities to join, volunteer opportunities to be had, and friends to be made. Although it is very important to learn the material, it is equally important to be well-rounded and adopt/re-adopt good techniques for keeping things in balance as medicine can be a high burnout profession.
I'm thinking about taking some piano lessons and maybe start playing Golf. Once in a while, I might play my favorite sport in the world, SOCCER ( I can't play all the time cos it takes a lot of energy, and I don't think I can study right after a soccer game.)