Six Flags has worthwhile rides, and low low crowds in May. It does not, however, have any "theme" to speak of, or 3D rides, or anything of that sort. Straightforward rides and such. Their coasters are on par w/ Universal's. Water park doesn't open til 5/19, but their water rides are good - water coaster is essentially the same as Krakatau at Volcano Bay. Get tix online if you go, they're much cheaper.
Canobie Lake is also worthwhile if it is open when you're there. It is in lower New Hampshire, and probably even closer to Boston than Six Flags. It has a few cool coasters and rides, smaller than Six Flags. Also low crowds in May.