Hand painted wood slices / Scenic Illustrations on Wood Slices by Meni Chatzipanagiotou. Granted these are painted & very beautifully done to but they would make excellent Wood Burning subjects as well.
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!