The perceptive and knowledgeable Londoner might look at the above photo and notice that it looks like this was taken from the top of the concrete jungle that is the Southbank Centre. And yet it looks like it's a lush and green park.
There's a reason for that.
The Queen Elizabeth Hall - part of the Southbank complex, next to the Royal Festival Hall - have grassed over their roof, planted a wildflower meadow, several trees, lots of turf, and raised beds with lots of veg in. It's an island of green in an ocean of concrete, with a view of the Thames too. I bet it's a bugger to water, but as you can see it's proving pretty popular.
It's free to visit, and they have a bar that serves a mean cocktail. If the weather warms up before it closes on the 4th September I'm definitely going back.