There was a recent article in the Sydney Morning Herald about a group or architects and environmentalists who want to make use of all of the unused rooftops in the city and turn them into 'green rooves'.
For those who aren't aware, a green roof is basically a rooftop garden that can be used as somewhere for food production or planted out as a cool place to hang out. They have very good environmental credentials providing an excellent insulation layer helping to evenly balance a buildings internal temperature. They have been used for centuries in Europe but for some reason haven't been taken up in Australia.
As Sydney becomes more densely populated such green spaces are going to become more and more important in providing respite for its inhabitants.