These are the neatest things I know of in Massachusetts:
The Mapparium- A three-story stained glass globe visitors can walk through. It's a whisper gallery and since it was built in 1935 it's a time capsule of political borders and country names.
The Glass Flowers at Harvard- these have been on my wishlist for years and years, so long that contemplating them doesn't fill me with wonder anymore, but I'm sure in person it will.
The Edward Gorey House (a museum) plus topical letterboxes.
Travel In The TARDIS- a 20 box Doctor Who letterboxing series carved by several different people (not linking because the box is restricted). I would get to find 20 stamps on just a 4 mile hike! In the New England woods! Fairy tale woods! I slightly hate the boxer who commented "Amazing series!!! Interesting characters, I think I may need to check this show out!!!" on this series.
Casey- a 23 year old research assistant who is always up for adventures (shown here at Yellowstone, WY) provided they're safe and well-researched. He is lovely, but he's not 3,000 glass flowers; he can travel to California perfectly well.