I used to do this with my 5th graders the day before a Spelling Test. It's easy, cheap, really fun, and did a great job cleaning off the tops of grimy school desks! :)
Begin by lathering some shaving cream (foam is better than gel) all over a surface (we used a light, portable serving tray). Start slow, by letting your kiddo use paintbrushes, or their fingers, to create drawings and designs in the fun, squishy medium. Then, be creative! Preschoolers can work on numbers or letters. Elementary kids can play a game of hangman with sight words. Tweens can refresh their memory on how to multiply and divide fractions...actually, I could use a quick refresher on those as well!
What ideas come to your mind??