If you've never been guerrilla gardening, you've gotta try it! There are groups in cities all over the world and it's a fun & rewarding experience.

I used to do this with the Toronto Public Space Committee years ago: you select a patch of neglected public or private land, and under cover of night, descend on the area with a few friends and a wagon full of bulbs and seedlings. When you're finished, an abandoned patch of grass, dirt, or gravel is transformed into beautiful flowers or even vegetables.