I'd head toward Great Island, which is next to Wellfleet. It's part of the CC National Seashore. I'd say that's where you'll find the fewest people. It'll be a good half day's worth of walking to see most of it; open woods of scrub pine, good views, nice tidal areas with billions of fiddler crabs to shoot at low tide, surrounded on all sides by beach. The best time is at low tide, but it's always a wonderful place to explore, no matter the tides. And nearby Wellfleet has nice shops and restaurants. Stop in at the CC National Seashore headquarters, get a map, and the rangers will tell you where the trails, etc. are.
The dunes out at Race Point in Provincetown are also worth exploring.
Merrill