Although I mostly ride Propect Park, whenever I have time, I extend my commute with a lap in CP. It trully is an amazing place. Like Steve, I get easily bored with laps, so I would suggest you also try to ride on the greenway, especially on the west side on Manhattan.

See attached for the elevation map of CP. I would like to give credit to the original poster, but can't remember who posted it here....
Anyways...
Yes, they still rent bikes at the boat house. Not sure what kind.
Yes, the pavement is better on the east side than the west.
If you really want to, you can ride through the winter; You can get "cheap" winter gear online or go to Paragon (not so cheap).

I still don't know why Slvoid hasn't chimed in yet...