Crayola Window Crayons!
In the interest of full disclosure, I will say that these things make greasy streaky messes on upholstery and carpeting. Not fun. But you know what is fun?
Turning all of your windows and mirrors into whiteboards.
We have used these for drawing, for spelling/writing practive, and for just plain scribbling. I've also appropriated them to leave morning reminder notes on the bathroom mirrors for the teens/husband.
They're my new favorite toy.