We just visited the Waterworks Museum, in Chestnut Hill.  If anyone in your family likes mechanical things (or science), it's worth the trip.

It's located in those beautiful buildings overlooking the resevoir, near Boston College.  Giant pump mechanisms, and some history if you're interested.  It's very casual, self serve type info, with an expert on hand for any questions.  It wasn't crowded on a Saturday, and it's fr-ee (fr-ee parking too).

If you're cabin fevered, like me, and you're looking for a cheap and easy outing, check it out: