The Children's Museum in Easton, MA is a small but fun museum that is perfect for the 5 and under set. I think it's best to go in the summer when they have their outdoor play area open. They have a nice garden area, as well as climbing structures and a very cool bubble tub.
The inside is fun too, though. It is a former fire house that has been converted into a museum, so it's on the small side. On the first floor there is a dress up area complete with make up and a huge mirror. The second floor has a Doctors office with all sorts of doctor tools, a wheel chair and more. There's much more to explore at this museum, although I don't think there is too much to interest kids older than 5 or 6. Check out their site for info on camps, birthday parties and special programs.