I don't know, I mean I was in the same position most of us were (accepted in the winter), so if you have nothing to do next semester (easy classes), and nothing really planned for the summer beyond being lazy and drinking, I'd suggest you start looking at "med school book lists" on Amazon, start reading reviews etc. If you have a job, start saving, and go traveling this summer, even if it's just a road trip with a buddy or alone here in the States.
Also, if you know what school you're going to go to already, it won't hurt asking for book list advice from current students. You could email the staff at that school, get a list of students who are willing to offer advice, and find out which books are worth buying, which aren't. It won't hurt getting a jump start on the purchasing-- getting to the old/used/cheaper books on Amazon before they're all taken. If you want an up to date edition, I'd wait to buy it, since you might by the most recent now that wont be the most recent when you start the class.
Some people say to get First Aid early, I don't know, I did, and it didn't really help. I got it, opened it, thought to myself "wow that's a lot of ****, the boards are gonna be ****ing terrible", and closed it. It remains in a corner of my room to this day (current MS1).
If you are REALLY bored, I don't think it would hurt to start looking over some subjects in basic sciences that you had trouble with in undergrad. All those nasty things that you glanced over (how many NADPH per cycle of glycolysis again?) and crammed for in undergrad are going to come back, so you'll inevitably have to cram again-- (v snares t snares jak stat smads etc). And really, if you want to get an idea of the pace you'll be at in med school, ask to look at your first block schedule, and get a feel for it that way. How many chapters of recommended reading per night, etc. If you want to do research too, you might want to look at faculty profiles of your school, find out who you'd like to worth with, what sounds interesting.
Like most have said, you're gonna be in the weeds most of the time in med school, with MORE than enough material at hand (textbooks you won't have to read through, websites you won't have time to review), so there really isn't a point to get started. You won't remember anything new. But I'd say it wouldn't hurt just to review some of the areas you know you could use a little more review in the basic sciences that you've covered already. Would it hurt to crack open a comprehensive anatomy book like Gray's Anatomy for Students and read from there? No, but the gains will be minimal. You won't sit there and make flash cards or take notes and memorize and test yourself like you would when you're actually in the class. But it won't hurt, beyond just wasting your time. I'd say your time is better spent reviewing the things you've covered already, but just to make sure the material is more familiar, so when you do go over it again in med school (an entire semester of an undergrad course in a week or less), you'll be in better shape.
Speaking of getting in better shape, I'd start working out, since I pretty much stopped when things picked up, and have been slowly wasting away into the body of a skinny, but unathletic 14 year old kid. You also might want to figure out some basic recipes for cooking, start planning how you're gonna be living, furniture you need/don't need, (should I sell my ps3 which I won't have time for, for beer money, travel money, furniture money, will that be enough beer money?).
Enjoy this time. This is the last time you'll be able to sit on your ass and be happy with all the work that you've done to get to this point, and not really have anything else to do.
