Actually I've never spent a summer in Boston although I've spent 4 dreadful winters here. You could drop in on summer classes in MIT. You could go to the beach - Crane Beach is pretty good, Revere Beach can be a little white trash. Unfortunately there aren't many good places to work out in the open

Also, YC is obviously in town, so find somebody who'll get you in on a few of the dinners and you get the speeches for free. :)