At Columbus Circle (southeast corner of Central Park) you can't walk ten feet without being asked if you want to rent a bicycle from all the hawkers roaming around. I finally gave in one day when I was waiting for my husband to get off work.
I expected the guy who got my attention a couple of blocks just south of Central Park to walk me to Columbus Circle where the big bike parking lot is located. Instead we walked past there, a couple of blocks east, to a narrow but deep shop front. He handed me off to the clerk where I paid my $10. I take that back, I had with me only $9! So the hawker pulled out dollar bill and helped me out! He probably thought, "Cheap tourist!"
Soon, then I was whisked onto a bike that had a large basket in the front for my handbag and wide handlebars. Did I get any instruction? Did I get a helmet? Did I get the phone number of the place in case I had an accident? Nope. Just a warning that it's not legal to ride on the sidewalks and a little push!
I had rented the bike for an hour and enjoyed every minute. There were several other bikers out, but it wasn't crowded at all. This was at around 7 p.m. on a week night. If you do this, just follow the same rules of riding a New York escalator: slow riders to the right, pass on the left.