Cape Cod isn't connected by land to the mainland, but it's not exactly an island, or at least wasn't until the Cape Cod Canal was dug in 1914. The 7-mile-long canal saves ships from having to sail an extra 135 miles around the treacherous tip of the Cape. The canal is also a great recreational resource.
It's bordered on both sides by bike paths that attract not only cyclists but also in-line skaters, power walkers and kids with fishing poles. On a sunny day it looks like a scene from a Norman Rockwell painting.
For the lowdown on the canal, stop by the Cape Cod Canal Visitor Center, near the Sandwich Marina.