Climber is ABSOLUTELY right, what you can learn now before school starts will be covered in the first couple days and you will feel like a chump for wasting time.
That being said, med schoolers are still in the premed mode and eager to be achieving, so here are some suggestions for the people who simply must be reading something:
http://www.studentdoctor.net/pandabearmd/
A former SDN guy and a good writer, go through the archives and you will find
some great write ups of what to expect in school and after.
House of God, Hot Lights/Cold Steel, or any other memoir of the medical experience.
Learn Spanish.
Goljan audio tapes
If you simply must do something that makes you feel all tingly and productive, listen to these tapes while doing something fun outside like cycling, hiking, or climbing. Exercise is a really good routine to get into now, before the crazy. And although you will retain jack-dippity-squat from the lectures, they will train you mind in how to think and study and practice medicine in the future.
And, Mandrake, please please please, enjoy your summer so that we can live vicarious.