You really don't need to study for step 1 after first year. But if you're going to freak out if you don't do some kind of preparation, here is a good compromise (won't take that long, you can still enjoy your summer): get the book First Aid for Step 1. Go through the sections that are relevant to what you've already done (anatomy, embryo, physiology, biochemistry). Completely ignore the bulk of the book which relates to path, pharm, micro, etc. If you do this, it will get you familiar with what kinds of 1st year stuff you will be expected to know (it's not all that much). Highlight it or whatever, makes notes in the margins if you're into that kind of stuff. Then, if you've already looked at it once in the exact book you'll be using to study for step 1, it will be super easy to cruise through it again when you're preparing at the end. The stuff in First Aid will seem basic to you, and it is... but although right now you probably think you'll never forget the brachial plexus or krebs cycle and that if you see it you will barf, I pretty much guarantee you will have to look at it again before the test.
Under no circumstances should doing that take more than a week (more like 3-4 days). Then you can really set your mind at ease. Relax and enjoy the rest of the summer, so you're set to go when you start 2nd year.
If you do what I've recommended and you STILL can't deal with not doing work, do the same thing with the BRS series on the first year subjects. Or pick one subject in BRS you feel particularly weak in. Pretty much unnecessary, but hey, whatever floats your boat.
bpkurtz