On a coworker’s recommendation I visited Great Island in Wellfleet all the way back in April. I’ve gone back since and it’s just as magical (and empty!) if you catch the right day in summer.
Great Island isn’t actually an island anymore – since the 19th century it’s been connected by a growing sandbar. You can read a quick history lesson here.
Here is the guide from the National Park Service: http://www.nps.gov/caco/planyourvisit/upload/FinalGGGreatIsl.pdf
They have a small parking lot that my GPS can’t quite figure out. If you want to visit and your GPS says you’ve arrived, keep going about another half a mile until you see the lot on your left. You’ll go over a small bridge with a great view just before you get there.