Was on Nantucket for an event called The Nantucket Project. Awesome event with a diverse crowd of people, all of whom make in a minute what I make in a year but ANYWAYS. Great thinkers, CEOs, philanthropists, etc. If you ever get a chance to go (and have $6k to drop on a ticket, I'd totally do it.
Nantucket is a cool little island. A lot of people hop over from Boston in the summer and work at the restaurants and bars. It's a ghost town in the winter as it gets smashed by wind and nasty swells. But during the summer it's one of the more beautiful places I've ever seen.
The beaches are reminiscent of Cape Cod, with huge dunes of sand leading up to relatively lonely shorelines. You can find nice places to hang out without being hit in the face by a wiffle ball every 15 seconds.
Be sure to bring a bike or rent one on the island. The stretches of bike trail span the whole island, you can ride tip to tip. Go for a ride and hit the LoLa Burger joint for one of the best burgers you'll ever have. Click here to see that monster.