depends on how long a ride you are looking for, but on Sunday AMs I like to do a loop where I start in Queens, head over the Triboro bridge to Randall's Island then take the pedestrian bridge to Manhattan. This will dump you on 96th street, you can now take the green path down to the 60s and crosstown, ride through Central Park and then head to the west side and take the green path loop through the Battery and back to the Willie B bridge and then down Roebling... forget all the streets but basically make my way to McGuiness and then over the Pulaski bridge and eat the Cave & Creek Mexican spot. (Yes, I live in queens.)
Maybe include Prospect park if you are in brooklyn. There is about a 3 mile loop on the outskirts of the park where you go as fast as you want on smooth pavement. You could also try kissena park if you have a track bike.