Begun in 1976, the Cape Cod Rail Trail is paved and in good repair. The rail trail extends 25 miles, connecting the towns of Dennis, Harwich, Brewster, Orleans, Eastham, and Wellfleet, passing beautiful forests, salt marshes, cranberry bogs, and freshwater ponds. This trail is part of the Massachusetts State Forests and Parks System and is a former railroad right-of-way.
Underpasses are a bit different here...
I was here in 2005 as part of an Elderhostel biking program, and here is a report from that week: Chuck's biking Elderhostel program on Cape Cod and Nantucket Island.
The Island Queen ferry will conveniently transport you and your bike to Martha's Vineyard, or you can rent bikes on the island.
Martha's Vineyard's bike trails run alongside the roads as seen in this photo taken between Oak Bluffs and Edgartown. This trail was extremely busy the Saturday I biked it, with dozens of bikers traveling to Edgartown and Chappaquiddick Island. Across the road (to the left) is a beautiful beach which runs for miles. Other bike trails extend to all sections of the island.
You can bike or walk the quaint streets to savor the flavor of the locale.
As on Martha's Vineyard, the bike trails on Nantucket Island are paved, are in good condition, and generally parallel the roads, offering safe and scenic routes around the island. A few slight hills break up the otherwise flat terrain, and since 45% of the island is preserved as open space, you often travel through scenic splendor. Again, the ferry accepts bikes or they can be rented on the island. Biking offers a far more intimate experience with the environment and culture of the islands.
Try to see Fred Rogers' "Crooked House" made famous on his tv show.