I spent the afternoon touring Roosevelt Island and Randall's Island on my bike. Nature in the middle of the East River.
FDR's Four Memorial at the southern tip of Roosevelt Island was the most interesting. I believe that the Island was known as Welfare Island for much of its developed life.
Randall's Island claim to fame is that it anchors the Triboro Bridge -- now known as the RFK Bridge. The play fields and picnic areas seem to be the attraction. Icahn stadium is the main event draw. The city plans a new pedestrian bridge from the Bronx to make access easier there. The Queens link is via 27th Avenue on the RFK bridge. Manhattan has a pedestrian bridge at 103rd and the RFK bridge deck at 125th or so.