no one has mentioned prospect park yet...you could always ride there.
but i think CP is WAY better
_there is also a velodrome in queens
_you could take the hudson greenway bike path up to 181 (exit the ramp) then ride on the street to 176th and go over the george washington bridge. this will give you good views of the city, and a weird experience of being on a huge bridge on your bike. if you want, you can go all the way over the bridge and take a left...going down the hill....this will take you to "river road" which is a beautiful and peaceful stretch of road with no cars that runs along the river for about 5-6 miles. really, really nice.
_if you do ride uptown...you can go to the cloisters museum (about 190th street). this place is sick. incredible.
_if you want to hit some good (clothing) shops, try famous friends in the lower east side, rogan nyc, and UNI-QLO