Hand painted tree rings - would recommend putting all tree rings on a board (which could be painted a coordinating color to add pop to the art piece. Really neat idea for after school kids. Might just do 9 rings in a square frame!
Plastic Bottle Vertical Garden: And on the super green end of the spectrum, you can use turn plastic bottles into an awesome sustainable vertical garden. (via Rosenbaum) How have you repurposed plastic bottles?