For our ninth (!) wedding anniversary, we headed Martha's Vineyard for the night. We took the boat over, moored up at Oak Bluffs, and stayed at a hotel in town. We rented a convertible and drove around the entire island. It was wonderful. It definitely gets crazy busy in the summer. We weren't able to book two nights because we waited til the last minute. If you can sleep on your boat and moor at one of the public spots, you'd be set! Definitely worth a visit, but maybe in the early Fall to avoid the crowds... we both still prefer Nantucket though.
Aquinnah Cliffs - a definite must see! Stunning. Be prepared -- it is a nude beach. C would fit right in :)
Oak Bluffs - not special.. although we did have some killer lobster rolls. We did hit up the famous Back Door Donuts. The idea is so cool (bakery open with fresh donuts 7pm - 7am ish). Unfortunately, not the very best donuts I've ever had. Still worth checking out... once.
Edgartown - SO cute. More Nantucket like, streets lined with white picket fences, hydrangeas, and gorgeous homes, all overlooking the water. Would definitely choose to stay here next visit instead of Oak Bluffs.
Vineyard Haven - cute little town to walk around. The Black Dog is here.