Having attended
WPI's Frontiers program last summer, I would highly recommend it. I have never been to any other educationalish summer program, but Frontiers was excellent. They have courses in different things (including robotics!), and there are many fun activities planned as well. You get to learn a bit extra about something during the summer, meet a bunch of cool new people, and have a lot of fun at the same time.
I think it is fairly popular among FIRSTies, because five who went last summer were at the UTC Regional today. There is (was?) also a scholarship for FIRST participants to did the robotics program at Frontiers.