Cape Cod is very bike friendly. There's a path that goes a long way (I took it from Brewster to Coast Guard beach). It's a very smooth asphalt surface, devoid of any significant hills. My only problem was with the people on it. Where I'm from, cyclists don't ride side by side when crossing other people. It can get annoying if that sort of thing irritates you. But then again, it's a casual ride and when you're on vacation, that sort of thing should get to you, after all, you're just cruising!
There's also a path on Martha's Vineyard that's nice, but the cost of getting there is a bit steep IMHO. We rode to South Beach (on Martha's Vineyard). No idea about Nantucket, haven't been.
Also, near Provincetown, there's a nice bike path that lets you go to a few beaches. Very very nice, although a bit short, but a must visit in my opinion. I don't remember the name, but it's right next to provincetown. The surface is a bit narrow and dated, but the view largely compensates and it's still much better than crushed limestone. There's a few sharp hills but nothing that should be a problem as they are relatively short. Also, some sand can get onto the track. Definitely not the place to go too fast!
here's some pics:
from
http://flickr.com/photos/jpmartineau...7601801672526/
Enjoy!