There’s an amazing variety of stuff up there.
First a piece from the Gothamist.
http://gothamist.com/2015/06/17/nyc_roof_cabin_with_yard.php
The temptation to put a small cottage or other structure on the roof can be overwhelming.
http://inhabitat.com/nyc/new-york-citys-hidden-rooftop-houses/
http://weburbanist.com/2012/05/23/the-fifth-facade-a-peek-inside-nycs-hidden-rooftop-world/
They even have bars up top.
Here’s an album of NYC rooftops.