Wal-Mart recently had these translucent plastic pencil pouches clearanced to 25 cents each. I snagged one in each color, knowing I would find a way to turn them into light table toys!
It didn't take me long to decide on making shapes! I grabbed Xander's wooden shape puzzle (similar to this one) and "traced" each shape with an Xacto Knife. Then I cut along the markings.
Ta-dah! A bunch of inexpensive shapes that can be used for sorting by shape or color.