Bike Nantucket. They have great bike trails around the island. My dad has biked around the island several times while visiting my sister.
I'll give a gratuitous plug for my sister, lookup Nantucket Sunset Sail
Nantucket Sunset Sail - Sailing Charters, Sailboats, Charter Business
If you talk to my sis, I'm sure she'll give you some pointers for Nantucket and the Cape. She has lived there for several years and was involved with the Hi Line Ferry. She also owns two homes on the Cape. If you take the cruise, tell her husband (the captain) you own a Porsche and you'll have a fun evening of conversation he's a car fanatic.