Well, we set out the other morning to make some blow paintings. What could be more fun. Put a blob of runny-ish water-based paint on a piece of paper. Give your kids a straw. (Explain clearly the difference between blowing and sucking, if your kids are little, or it could be a much too exciting morning). The kids blow through the straw and off goes the paint into beautiful unpredictable forms. Of course, your paint consistency has to be just so. Ours was too runny at first, and the kids mostly sprayed each other.
Then our paint was not runny enough, but perfect for drip painting — Put a blob of paint at one end of your paper. Tilt the paper so the paint drips down, then turn the paper left or right as the paint guides itself around the page.
Then, of course, one of my kids accidentally sat on her drip painting, luckily folding it in the process, and made a Rorschach painting. We all thought it looked like a butterfly. So many options on an early Fall day!