My husband and I go to Truro Mass. in the Cape. It's at the veeeery end of the Cape right before Provincetown, but we go for the beaches and the whale watch cruises since P-Town is the closest to the Stellwagon Bank where a majority of the humpback whales feed till October. Its's now also an arts community originally settled by Portugese Fisherman.
We stay every year at these cottages
http://home.comcast.net/~coelho.m/
They are very clean, and families tend to stay in this area as opposed to P-Town which is an artsy community. Its fun to walk around P-town at night. There are lots of quaint shops, cobblestone streets,art galleries, great restaurants and a very colorful harmless crowd.
Then during the day, they can also visit the beaches which are quite breathtaking. The water is COLD though. We spent a day once just hanging out on the beach watching a family of seals fishing for giant snappers.
There's also Martha's Vineyard and Nantucket. You have to take a ferry boat out there, and its very beautiful, but very posh and expensive. There are lots of Bed and Breakfast places, but not sure about weekly or daily rentals. Most of the houses are rented for the season.
In Boston, there's the Freedom Trail- just do a google search. Lots of things to see along that path, historic cemetaries, lots of history, museums etc.
I woouldn't so the Freedom Trail on a Sunday as its a strict Protestant area and everything closes up on Sunday- hence we called it the Starvation Trail because we finally found a reastaurant open in the Little Italy section.
Can't remember where we stayed, it's been a long time since I ventured into Boston.