I'm on Cape Cod so..
Spend a day in Provincetown shopping and sight-seeing
Tour Cape Cod Lighthouses
Take the shuttle to North Beach in Chatham to swim with the seals
Ferry over to Nantucket or Martha's Vineyard for a day trip
Hit the Wellfleet Drive-In and fried clams before the show at Arnolds
Bike the path nearest the National Seashore or do the whole bike path

Spend at least one day on a bay side beach from high tide to low tide
Go deep sea fishing out of Barnstable Harbor (or a whale watch with kids)
Meander along Route 6A checking out farm stands or antique shops
I could go on and on but what I wouldn't do is...
Beach on the southside anywhere on the Cape (and if you don't know the difference, you may very well end up there)
Meander along Route 28 at any time in any town