We love the new trend of "greening" one's roof by replacing shingles and tar with vegetation. Think of how much extra space has been added to this building in the Embarcadero, just by utilizing the roof?
Adding a green roof isn't usually an option for renters, but if you have safe access to the roof and a lenient landlord, you might be able to do something similar. Add a bunch of planters and some chairs, and you've got an outdoor sanctuary in the sky. Having such a space might even increase the feeling of community in your building - just as long as the footsteps don't bother the top floor tenants.
Top image: Pbo31; Bottom image: Democratic Underground