Enjoy your time off, tavel, work, do whatever you want to do. I just worked the summer before coming down because I needed to, and it kept me from being bored.
Before school starts they'll probably send out the book list and maybe some of the required reading for your first semester in PBL, but I don't know anyone who really did any reading/studying over the summer. Just be ready to jump into anatomy.
No real surprises here for me. Your first 10 weeks will be anatomy so it'll be a lot of lectures and lab time. And you'll slowly start PBL and it'll take time to get used to the "process" so don't worry if you get stressed and/or frustrated with it, I think everyone did at some point. There's nothing real technically hard in med school, it has a lot to do with the amount of material and being able to stay on top of stuff and manage your time well. seriously, if you can keep from getting behind in you reading and just keep on chugging through the material you'll be ok.
So I'd say just enjoy the time off.