I've done summers at Central Piedmont Summer Theatre (Carlotte, NC), Maine State Music Theatre (Brunswick, ME), Walnut Hill Summer Theatre (Natick, MA), Yale Institute of Music Theatre (New Haven, CT), and Second Stage Theatre (New York, NY). They were all very different jobs but I enjoyed them throughly. If you'd like specifics, feel free to pm me.
As a recent grad school graduate, I would say it probably would have been smarter to have spent my summers focusing on what I wanted to do after school. I have since moved to NYC and if I would have decided that sooner, it would have been smarter to concentrate my energies here, developing contacts and learning the area. My school did mostly classics and new work, so I would try to mix it up and work on some musicals in the summer. I did do a new musical worship that was fun and refreshing. I don't really regret my choices. I developed some amazing collaborator relationships and the experiences really grew me.
Best of luck!