Governors Island
New York City recently (re) acquired rights to Governors Island, which is a small, unpopulated, and mostly undeveloped enclave used as a US Coast Guard station for some decades. It is now being turned into.... well, no one is quite sure yet, but so far a lot of people are enjoying taking the free bike-friendly ferry service from Manhattan and Brooklyn to it on weekends for some traffic-free biking and exploring.
Governors Island Preservation web site.
This link goes to a full set of photos taken by me on Saturday.