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.
Have fun...they should be 4 great years.