There is nothing you can do the summer before med school that will remotely affect your residency chances. Do some traveling. spend time with family, friends. Watch lots of movies. Go to the beach. You won't have nearly as much time for any of this once school starts, so the best thing you can do is pack in all the fun you can. Basically like carboloading before a race -- you are going to need to show up refreshed and not burnt out. The WORST thing you can do is try to learn first year before first year starts. Stay away from anything academically related. You will be doing that way too many hours/day once classes start and the one thing you don't want to do is show up burnt.