Collins Foundation.
Hudson MA has one of the best collections of historical airplanes and military ground vehicles in the United States, most of which are theoretically operational. They have (I think) one of two privately held M1 Abrams tanks in the whole country (but don't fact check that).
The American Heritage Museum at the Collings Foundation featuring the Jacques M. Littlefield Collection explores major conflicts ranging from the Revolutionary War until today.
www.americanheritagemuseum.org
Their advertising is not great; a shocking number gun and defense geeks literally living in the same town don't know they're there. But it's a better collection than, say, the actual US Army has at Ft. Belvoir.