I envy the people who personally came across the Yarn Bombings in Prague or other places, but at least I have witnessed several times the guerrilla gardens that try to reclaim public space.
This is a rather colorful example, although not from Prague:
This is really a refreshingly different form of street art than just spraying some scribbles on a wall. The creators have done considerable work at visible places. During the warmer seasons of the year, they go guerilla gardening.
Not in Prague, but also worth a look are these works.