I just took step 1 a few days ago so make of that what you will. I'm not sure that brushing up on anything is really worth your time in the summer. In the absence of an upcoming test, your brain won't be as inclined to absorb and integrate the details. You will spend your time passively learning and then promptly forgetting the material you just covered within a matter of days. I also found that it wasn't until March/April of 2nd year that I really knew enough about every subject to integrate and solidify the details that are relevant to boards. By that time, you will have a much better grasp on what details are worth remembering for boards, or even studying in the first place. You just don't have this knowledge base when you've only finished first year. This makes studying over the summer a huge waste of time when you could be relaxing and recuperating for the coming marathon that is 2nd year/step 1. If you want to be productive, try and hop onto some research, read the news, learn how healthcare insurance/industry works, etc. Anything but study the different cytoskeletal proteins in cells, because I can assure you, that stuff is useless--you just don't realize it yet.
That being said, if you already started systems 1st year (we had completed MSK/Cardio/Respiratory) and haven't done the relevant questions from one of two question banks (Kaplan/Rx), you can do those. But that's it. Seriously. All you need to worry about until about December of 2nd year is having done the 200 or so board questions from either Kaplan or Rx for each system you've done. Once December comes along, start doing the other qbank on random timed. Finish that by about March. Start UWorld. Finish first pass by May. Start 2nd pass. Take boards in early June. This will undoubtedly produce a score that you desire, and none of it requires studying during the summer. Remember, this is a marathon, not a sprint. Enjoy life a little. We all know that it's far too short to spend a summer inside studying when you will have nothing to show for it.