Veering away from pure Vegetated Architecture (a bit), let’s hone in on some examples of usable green space – particularly in the realm of sports. 
:: images via Marshu
While a tennis match between Agassi and Federer atop the helipad of a Dubai hotel sounds lovely (and a bit dangerous?) – there’s a number of more reasonable approaches to rooftop sport that actually include places for the crowd as well. Here are a few. A project with from Jaroslavska, Moscow by Ofis Arhitekti incorporates mixed use with a high-grade football pitch.
:: image via Dezeen
Ok, it’s not rooftop, but a fun design anyway from Will Alsop for a London cricket ground, with a ridiculous building, per usual.
:: image via WAN
The project I always turn back to when looking for great examples of on-structure sports is the quasi-rooftop waterfront of Pier 40 in NYC that has parking and multiple soccer fields.
:: image via The Villager
:: image via A VC
Or a variety of rooftop golf scenarios, with a more encompassing and fictional version (top) from Work AC, previously shown here – and a small-scale synthetic version (bottom).
:: image via NY Mag
:: image via Synthetic Turf Illinois
The idea of play can emcompass non-green, capturing usable spaces for sports courts and playgrounds, such as these rooftop playgrounds in NYC, both new (top) and old (bottom).
:: image via Unity Surfacing Systems
:: Early 1900’s era rooftop play – image via America’s Library
Guess good ideas never get old… get up there and play.
Riverbank State Park in Manhattan is a great example of rooftop athletic space. On top of sewage treatment plant there is an olympic pool and track with artificial turf, a carousel designed by community children, and various playground and picnic facilities and green space.
http://en.wikipedia.org/wiki/Riverbank_State_Park
http://farm1.static.flickr.com/22/28100309_7d9cc6a4fa.jpg?v=0
Thanks Bryan for the heads up on Riverbank State Park… truly one of the best examples of rooftop play…