Since 1999 I've gone to Cape Cod every spring or fall. I stay at Sandcastle Condominiums in Provincetown. (See the Sandcastle journal entry.) Sandcastle is located right past the town line, so it's on the outskirts of town. It's a quick bike ride or drive into the center. During the late spring, summer, and early fall, an inexpensive shuttle runs along the road and into town and to the beaches.
The information below is about a trip in late June where my son (8 at the time) and I spent two weeks at Sandcastle.
At this time of year, there were many children at the resort, which was nice for my son who always had someone to play with at the indoor pool. We also enjoyed walking on the beach at low tide when we could see many little sea creatures including hermit crabs, lady crabs, and even a tiny pipe fish. In late June the water was warm enough to lie down in with our masks and snorkels.
The National Seashore areas are also worth visiting. The Outer Cape is gorgeous and wild with high sand dunes and wonderful areas for bicycling.
We went into Provincetown and enjoyed walking around. We were there during the weekend of the Portuguese Festival and saw some of the entertainment and the parade. Provincetown is a very tolerant and diverse community. It is known for being gay/lesbian friendly.
We enjoyed buying freshly made saltwater taffy, shopping at Puzzle Me This, an excellent shop full of games and puzzles, and walking the town pier to look for sea creatures in the water.
On other days, we went to the Wellfleet Bay Wildlife Sanctuary, which has excellent and very reasonably priced programs for adults, children, and families as well as children's day camps in the summer. The National Seashore also offers many programs for adults, children, and families. The National Park System has a Junior Rangers program for children, which is available even in the off-season. Another day we went to Brewster to the Cape Cod Museum of Natural History (summer camps also), which has aquaria and a touch tank. In Brewster we also visited Nickerson State Park and swam in freshwater Flax Pond, which was full of tadpoles. Luckily we had our masks and snorkels with us.
When you're on the Cape you can get a library card for $5. It's good at all the Cape libraries. A card allows you to borrow books and to use the computers for checking email. Another great thing you can do with a Cape library card is check out a museum pass. I was able to borrow a pass to Plymouth Plantation that saved us about $20 on admission fees. It only cost $3 for my son and me to visit Plymouth and the Mayflower reproduction.
I wouldn't recommend a trip to Plymouth Plantation if you are only on the Cape for a week. There is enough to keep you busy without leaving the area. But for a trip of two weeks or more, a trip to Plymouth is quite easy to do.