We've stayed at Sweetwater Forest a few times. It's centrally located but as for kids' activities - ours are grown.
Wandering around Provincetown is an experience. We've done a few whale watch trips but the novelty has worn off for us.
+1 for the Cape Cod Rail Trail - a paved multi-use trail we enjoy biking on. Our only issue is with people pretending they're in the Tour de France and yelling at YOU to look out. If you make it that far to have to stop at Arnold's in Eastham for lunch. They have an Airstream trailer equipped as as a restroom right off the bike trail.
DW enjoys the Cape Cod Natural History museum in Brewster. Nice exhibits, and you can walk the nature trail down to the ocean.
Also in Brewster is the General Store and ice cream shop. Lots of novel and unique stuff in the General Store.
We took the ferry to Nantucket ($$$) and spent the day there. IMHO the privately-run whaling museum there is better than the NPS one in New Bedford.
The Salt Marsh Visitor Center at the National Seashore is nice. We like taking the guided canoe tours of the marsh. And you can bike to one of the beaches from the parking lot.
The last time we were there I did some research and got the truck equipped so we could do some beach driving. Air down the tires, put it in 4 X 4, drive out on the beach, and spend the day at Race Point.
Traffic - avoid arriving on a Friday night or Saturday, and leave early on Sunday morning, otherwise you run into the weekenders. And there's a tourist information center on rt 6 after to get to the Cape.