Color furniture with vertical rainbow stripes like this Zoe Murphy chest of drawers. You can try strips of coloured tape for a non permanent option, assuming you aren't like me and have the patience to rip off the tape carefully afterwards.
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!