The other day we bought a bunch of them. Monkey1 and Monkey2 actually used some of their own money, which I believe was left from Christmas, and got two each. Then I bought one for Monkey3 so he wouldn't feel left out.
Once nap-time started, I got out freezer paper. I folded over the edges just a bit, to keep it from rolling back up. Then I squeezed gobs of paint directly onto the paper. (I have also used a rectangle of cardboard, such as could be cut from a cereal box, for a palette in the past.) They shared a cup of water for rinsing brushes, each had their own rag for drying brushes and taking care of minor messes, and generally had a ball and kept themselves entertained for about an hour. That definitely works for me!
A word of caution: If you try this at home, make sure you put the paint away before nap-time is over. I didn't. When Monkey3 woke up and found it, he unscrewed the cap off one of the bottles, and carefully poured it all over the rest of the bottles and his high chair.
For more good ideas, visit Works For Me Wednesday, hosted by Kristen at We Are THAT Family.