I have boys 6 and 4 and aside from many of the items already posted we really enjoy the following below that maybe arent as well known. We are in Harwich and spend a lot of time there.
FWIW A&W is not what it used to be and is pretty expensive actually except on Tuesdays its .99 chili dogs.
Heritage Museum
http://heritagemuseumsandgardens.org/ far down from you but pretty cool place
Natural History Museum
http://www.ccmnh.org/- dont forget beach / low tide gear nets buckets etc
Threaded Mode