Following the advice above, don't do anything this summer. And during your first year, don't do anything with the intent of "preparing for the boards."
However, a lot of people do the following during their first two years of med school, and it might be something to keep in mind:
Buy First Aid for the USMLE, BRS Physiology, and Rapid Review Pathology. These are good resources that many students use for board preparation, and are also very helpful in reviewing for various system and/or subject tests. The goal would be to read relevant chapters/sections along with school to help solidify the things you are learning - not as a primary source of information. And by reading through them during coursework, you will be much more familiar with them when boards comes around.
Keep in mind that when I read through a chapter of RR Pathology for a particular system block, it takes about an hour or so for 6 weeks worth of class, so what I'm suggesting isn't putting a great deal of time into utilizing these resources. You'd be spreading them out over two years, with the primary goal being to know what is and isn't in them when you are ready to start board prep way down the road. Also, a lot of people find BRS Physiology and RR Path to be particularly useful at shoring up general concepts during their classes.