Crayon resist painting is simple and fun to do. I remember doing this when I was younger.
Draw a "secret message" with a white crayon and have your child paint over it with watercolors to see the message appear.
This is a neat activity if you are working on letter of the day, shapes, introducing a new theme, etc. Our paints weren't watery enough or something. It took B forever just to cover the little areas he did paint and then he was done. Also, he didn't really buy into the whole "secret picture" thing I tried to pull. He could see the white crayon outline which is why he only painted on those areas. Next time I will only use 1/4 of the sheet of paper and maybe it will come out better.