+1 on the Cape Cod Rail Trail! It's a paved milti-use path that runs through Nickerson State Park. Check out the Cape Cod Museum of Natural History in Brewster too. And the Brewster General Store and ice cream shop.
We've been to the Cape several times and stayed at Sweetwater Forest in Brewster. Can't comment on the other cg's.
On our last trip I did some research and got our truck outfitted so we could drive on the beach at Race Point. $50 for a 7-day pass and we had several relaxing days on there.
We did a day trip to New Bedford to see the whaling museum but were both disappointed. Both of us felt the private museum in Nantucket was better. Caution - If you go, the ferry ride can be expensive for a family.