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!
i do have lots of cork left over. DIY Cork Memo Board: Inexpensive cork transform into a contemporary office bulletin board. Candy-color pushpins organize invitations, hold business cards, and show off interesting ephemera. (Go to bhg for the how-to.