Myself and my wife went on a day trip to Cape Cod when staying in Boston once - went to Provincetown. Its a nice pleasant seaside type of town.
One thing that should be pointed out is that the area is almost exclusively gay. When walking around the town, you could tell who the day trippers simply because we were they only straight couples or families in the town. You will also have to get used to the locals double taking when they see a straight couple walking down the street hand in hand (some look at you as if you have two heads). If you are not gay and you go there for a short visit and you know what to expect then you can regard it all as a bit of a novelty and have a good laugh.