Cape Cod National Seashore is amazing--truly majestic and well worth the experience.
Here's the official park explanation: it comprises 43,604 acres of shoreline and upland landscape features, including a 40-mile-long stretch of pristine sandy beach; dozens of clear, deep freshwater kettle ponds; and upland scenes that depict evidence of how people have used the land. A variety of historic structures are within the boundary of the seashore, including lighthouses, a lifesaving station, and numerous Cape Cod-style houses. The seashore offers six swimming beaches, 11 self-guided nature trails, and a variety of picnic areas and scenic overlooks.
We went from one beach to the next, each as impressive as the last. This is a bit of a drive from the mid-Cape, where we were staying, but it is so much more pristine, majestic, and gorgeous that you can't even compare traditional beaches to this protected sanctuary. You must go and see it. It encompasses some 30 miles of coast that was placed into federal protection by JFK, a Cape Cod enthusiast. He knew what he was doing.
Visit the website to plan your trip, as you have many beaches to choose from: http://www.nps.gov/caco/index.htm. But make sure and go.