I'll throw yet another option into the mix:

Concord and Walden Pond. Very close to Boston, although will require a combination of train (MBTA) and taxi/uber (unless you're into long walks). Walden Pond off season is a beautiful pond to walk around, especially if you've read Henry David Thoreau and it's not too cold. And the town of Concord is a nice, quaint, old town to walk around.