I was cleaning out my bookmarks and came across this link I had been saving for a while. If you garden you may get a kick out of what these crazy green loving Europeans are up to.
I haven't done anything on this scale but I have thrown handfulls of extra seeds I had in neighbor's yards who have nothing growing and in an empty parking lot near my house. Mostly the Asclepia Syriaca that I find near the rail road tracks, but it's better than nothing.
Check out Guerilla Gardening by clicking the link above. They have lots of neat info and cool pics of the work they do. Maybe they will inspire you too.