I'm the first to admit that there are times my kids just need to be busy and let me get things done. We've used this activity since they were old enough to walk. And in our house that was very early (9 months). And I don't really what age this becomes uninteresting for them because my 6 1/2 year old still loves it!
What you need: a paint brush (any kind), a bucket or bowl, water. That's it!
Send them outside and have them paint the house, the fence, the concrete, their bikes, whatever they want. Afterall, it's just water and probably can't hurt much. Make sure you explain to them what is off limits. Like my car is offlimits to the kids "painting." I don't want hand marks all over my windows should they reach that high.
My favorite is when they paint the concrete. It changes color when water hits it so they really think they are painting :). Try it! It really works.