As I mentioned before, I visited Governor's Island last weekend and it was lots of fun. It's only a 7 minute free ferry ride from lower Manhattan and offers some gorgeous views of the City. There's lots to do, including renting bikes and taking a ride around the island. You can even rent a bike for a group!
I read this description in an exhibit in one of the buildings on the Island: "Governor's Island, situated at the intersection of the East River and the Hudson River between Brooklyn's Red Hook and the tip of Manhattan's Battery, is an unexpected twilight zone, 172 acres of public land on which only a few have ever set foot. For the most photographed city in the world, one that we locate through its images and films, it is remarkable how unfamiliar Governor's Island remains to the inhabitants of New York. That will change soon as the Island, once the exclusive domain of the U.S. Coast Guard personnel and their families will open to the public for the first time in over a century."