Favorite beaches: Truro, Truro, Truro! Any of the ocean beaches are great and were uncrowded in early September when I was there. We were staying in Provincetown (not high on my list, but some nice beaches, too) and wanted to get out of town and see some nice beaches - someone recommended a few in Truro/N. Truro and we checked them all out over a few days - wonderful! Have been since looking at rental properties for September and hope to do a family reunion there one year.
Can we consider Martha's Vineyard the Cape? If so, then that's my favorite destination. Stayed for 10 incredible days in late September a few years back at a great hotel in Chilmark. A beautiful island - ran, walked, hiked, biked, swam and ate like a king. My fantasy is to buy a little place nestled above Tea Lane ... ha! I'll need to work a few more years to make this dream come true.