Cobies right down the street from Nickerson. Great clams and burgers, black & white frappe, but tourist priced. 36 hours I wouldn't bother spending 8 of it going to P-town, especially with young kids. Too many things to try to explain.
Just go to one of the Dennis beaches, Corporation or Chapin. See if Nickerson has a day pass to a Brewster beach. Also, if you saw the movie "The Finest Hour", the actually boat in that rescue is on display in Rock Harbor.
Finally, if you can't get to any beach, PM me. You can park in my driveway and walk to one of the best beaches on Buzzards Bay.