I know lots of people think Normandy Farms is a great campground but I think is is a bit of a travel to the places the OP would like to visit, Boston, Plymouth, and Cape Cod.
Maybe a few days in Boston Plymouth area then a few days on the Cape would allow more seeing and an hour or two less driving.
Just my thought ....
And a good thought it is. It's 112 miles from Sandwich at the beginning of the Cape to Provincetown, at the end of the Cape. Something at the "elbow" as someone suggested is a great idea for Cape exploration. For Boston Minuteman KOA and Winter Island are probably better than Normandy Farms but Normandy isn't a bad options. Plimouth Plantation is well worth the visit. The rock is just a rock, really.