- Take the ride on the Roosevelt Island tram. Once the site of a state mental hospital and other hospitals, Roosevelt Island in the East River is now a city unto itself, but with almost no cars. You can see the creepy ruins of the old asylums and the Smallpox Hospital (photo above), and you can visit the FDR Museum if you want to and the Four Freedoms Park. (Not an FDR fan, myself, but many are.)
- The New York Transit Museum (h/t reader). New York first had urban mass transit in 1868 (Els) and underground since 1904.