I was very inspired by the book called The rurbanite, by Alex Mitchell. (you can find what I wrote about her on my bookblog here)
In this books she writes about creating a greener and better environment in our cities. By gardening, not just in our gardens and on our balconies and windowsills, but by making plants grow in the places you would not expect them to grow.
You can put pots on your doorstep and on the stairs of your house. But you can also go a little but further. That is a form of guerilla gardening, claiming bits and pieces of the city to grow a few plants. By a tree, along the pavement. Don't ask permission, but if you do it right, the counsil will probably let you get away with it.
When I was in Amsterdam a couple of weeks ago, suddenly I saw examples of guerilla gardening everywhere! When you are looking around, you see how people tried to make the city greener in all kinds of creative and beautiful ways.