You’d probably be better off taking a little island hopper and flying. Just a suggestion unless you really want to spend all day in transit.
If you go to Nantucket, you have to get the following T-shirt: “I AM the man from Nantucket.” They also have them for pets: “I AM the dog from Nantucket” etc.
I haven’t been to Martha’s Vineyard but have been to Nantucket twice. Its very touristy and if you enjoy shopping, you’ll have a great time.
How soon is “upcoming”? It’s getting to be off-season on the Islands and they may be a bit quiet with some of the restaurants and hotels shut down. I have been to the Vineyard in September and it’s actually rather nice not to have all the tourists around. As I recall most of the trees are scrub oak, so there probably won’t be a lot of autumn color. The ferry ride is kind of fun.
At best, I’ve always found the Vineyard to be a pleasant place to get away from it all – it’s reasonably quaint, lots of New England coastal atmosphere, but it’s not visually spectacular except maybe for the cliffs at Gay Head (OK folks, snicker away…) But you could accomplish pretty much the same thing by driving up to Gloucester or Rockport and it would be a much shorter ride from Boston. And if you wanted to take a boat ride, you could go whalewatching.
As a native of Rhode Island, I have to recommend Block Island. It’s tiny, but very beautiful. Like the larger islands, much is closed in the off season, but you may prefer the lack of insane hordes of tourists. The Mohegan Bluffs are very beautiful, and I also recommend hiking the trail system maintained by The Nature Conservancy and others. For that matter, the Conservancy rents a very sweet little cottage, walking distance from the ferry, relatively cheaply. (No heat or indoor shower, though).