Plant Trees lots and lots of them. Go Guerilla Gardening its great fun.

You take a largish sheet of paper and using Origami techniques you turn it into a square cube about the size of a tennis ball is good. (We did it at school many years ago and filled them with water and dropped them out the top of multi-storey bulidings on passers-bye. ) Instead of filling it with water you fill it with a bit of soil/compost/seeds/bulbs etc. Then the fun bit you wander around to an ugly unkempt area and bomb it with your cubes. Best do it under the cover of darkness. The seeds and bulb grow giving colour and beauty to the city. It takes a bit of work, planning and patience but the result is worth it. The more the merrier.