I cannot seem to track down my inspiration for this idea, so I can’t attribute it. But, we had a great time! Dylan liked moving the paint around, and I liked that there was no mess!
All you have to do is drop some paint globs down into a Ziploc bag, tape it up to the window, and then (if you have my kid!) sit nearby to make sure un-taping it doesn’t become more interesting than the “painting”.