Block Island - that's stuck in nowhere for 2 months!
Cape Cod is OK but you aren't going to see much without a car.
Boston - Constitution marina is a good spot for sight seeing as you can use public transportation, but it will be noisily with the highway next to it etc.
Salem - Hawthorne Cove Marina, I stay here because of the location mostly. Easy walk into town, train station into Boston, ferry
into Boston, a lot easier to do some sailing compared to the amount of time to get in/out of Boston harbor (which has a lot of islands and is 20 miles sail away). You could get a mooring
that includes launch service
that runs 8-10 during the season.