I'd say prepare some of the generic essays (why MBA, career plans, leaderships, etc), select a set of people to write your letter of recommendations (& have them committed writing the letters), research schools thoroughly... These should keep you busy for a while.
Above all, enjoy the summer. You'll have a stressful year ahead of you
yes, you can definitely visit schools before interview, and I highly recommend it. You have to check with the schools and see if they offer such programs in the summer. Some offer them along with the on-campus interview, while others have it separate from interviews. In any case you can always just go and visit unofficially (to check out the locations and all).
Come to think of it, it may be tricky to visit in the summer because most students are away for internships & you probably won't be able to talk to many of them. Just something to keep in mind.