Not sure of your timing, but water excursions include whale-watching tours or pirate-themed cruises out of Plymouth Harbor. Depending on your grandkids' ages and interests, lots of pilgrim-type activities in the form of historical sites and museums, including a replica of the Mayflower and a few living-history exhibits like the one at Plimoth Plantation. If the kids are younger, the town's sheriff's departments runs a frew petting zoo!