Mix a dollop of kid's craft paint (washable is best) with several tablespoons of water.
Spoon scoops of watered-down paint onto a sheet of paper. (I always encourage my kids to chose two primary colors so that they can see how secondary colors are made.)
Using a straw, blow each color blob in all different directions to mix the two colors together. If the paint does not seem to "blow" very easily, add a few drops of water to each "blob."
What is your favorite "go to" boredom buster?