It has been many years since we explored the mid-cape and P-town area but we do take day trips to the canal for lunch. (only 1 1/2 hours from home) Mid-week is the best time to travel to and on the Cape.
Crowe had some great suggestions. Coast Guard beach is considered one of the best in the area. Park at the Visitors Center and take the free tram to the beach. The lighthouse at Chatham (on the "elbow" of the Cape) gives a fantastic view of the ocean and has parking that should handle a mid-sized C rig. It might be an interesting ride past it going to/from the beach. I don't know if you can still walk down to the sand from there.
Is does seem that if you are going to the Cape a ride to P-town completes the trip. Rt. 6 is good road ALMOST into the town. The kids might like to climb the 252 feet to the top of Pilgrim Monument at P-town. The view from there is fantastic.
Enjoy