This plastic pad comes with a refillable felt tip pen that holds plain water. When you draw on the pad with a pen (or a brush loaded with water), it produces a pleasingly solid blue line. When the water dries, the line disappears completely. My 3-year-daughter plays with it almost every day. $18.99 on Amazon
Mark Frauenfelder is the founder of Boing Boing and the editor-in-chief of MAKE and Cool Tools. Twitter: @frauenfelder. Come and hear Mark speak at the ALA conference in Chicago on July 1.