NYC may not be the best for biking but the diversity & excitement beats any other city. Mountain biking really sucks if you live in the city, I used to drive 35 miles each way to Ringwood for some decent challenging trails.
On the plus side, on a nice weather day in Central park, you can get whiplash checking out all the cute girls in bikinis by Sheeps Meadow.
BIking up to GWB out to 9W is fun. There's some good rides in Western NJ by Hunterdon county