Totally agree with adding Shelburne Museum to your list.
Lowell National Park --- birthplace of Industrial Revolution, looms, quilt museum
http://www.nequiltmuseum.org/ . The NPS does a wonderful job explaining the area and they have a great tour with a canal boat ride, visit to the factory and boarding house. Really makes you feel for the young women and kids who were employed by these mills.
http://www.nps.gov/lowe/index.htm
Willard Clock Museum in N. Grafton, MA.
http://www.willardhouse.org/ Just visit their website and if you fall in love with the photos then it is a must stop. The tour will take at most 2 hrs and when I visited with my elderly parentsthe docents really geared their talk to older folks, very nice visit. I returned a year later with my boys and they did not care for chatting with my boys---very well behaved and listening boys--but the docents were not engaging at all.
Higgins Armory, Worcester, MA --- The Higgins family was quite wealthy and one loved armory. This unique collection is a must-see if anyone in your travelling party likes medieval armor.
Battleship Cove, Fall River, MA --- If you are into seeing historic naval ships it has a few. From here stop in to see Submarine Base in New London, CT There is the USS Nautilus, first nuclear sub, and a few others there.
http://www.militarynewcomers.com/NEW..._historic.html
And if you like aquariums the New England Aquarium in Boston is the finest in the country IMHO.
Carole, (lived in middle-Mass for 16yrs)