Saw this idea on Pinterest and thought it looked like one both boys could enjoy. As a bonus, it was easy and the mess was minimal.
First, I traced the bottom of our salad spinner onto some card stock and cut.
Then I put the card stock in the bottom of the salad spinner.
I had the boys pick a color of paint and squirt a bit onto the paper. We closed the top and took turns spinning.
It was fun to do one color and then see what turned out, but we could have done several colors at once.