In the two years that I've lived in NYC I've explored many areas and towns around the area but somehow hadn't made it out to Roosevelt Island. Roosevelt Island is an island in the East River right under the Queensboro bridge. It runs from 85th street on the UES down to 46th street. To get there you can take the tram over the Queensboro bridge.
The tram is only one of three commuter aerial tramways in the US and you can use your Metrocard to get on it. It's really fun to ride over the bridge and see the city in a completely new way rising up from 2nd Ave.
There is a bus on the island that takes you around but you can also walk to the main attraction-- the Four Freedoms park to the south. Here you can see the UN building across the river as well as the three main buildings of NYC all from one spot: the Empire State building, the Chrysler building, and the Freedom Tower.
And then there's the smallpox hospital that was abandoned in the 1950s.
Roosevelt Island is a great place to visit if you're someone who doesn't like the hustle of exploring busy places. It was a really nice way to spend a quiet afternoon taking in the city. :)
Also see: Rollerblading on Randall's Island. :)