All you have to do is draw pictures, shapes, or messages in a white (NOT washable) crayon.
Then let your toddler draw all over it with markers, watercolors, or finger paints. (It does not work with other crayons). The shapes will magically appear. I like the let Benjamin paint with thin strips of a sponge instead of a brush.
For older kids who can read, you can write special secret messages to them that they have to uncover or decode. You could even use this idea as a treasure hunt where they have to go around the house to find each clue, use a marker to reveal the secret message that will lead them to the next clue until they finally find the prize.