"Magic" picture created with wax crayon and food coloring.
When my children were much smaller and were fretful due to being inside in rainy weather or just needing a little fun, I would do "magic" pictures with them.
Take a piece of white paper and using a white wax crayon draw a simple outline picture of anything - a ball, tree, cloud, house, dog etc.
Then let your preschooler color wash over it with water color paint or food coloring in water.
The picture "magically" appears!