Paintbrush and water.
I put some water in her plastic bucket. (It was a bit muddy from our gardening project.) I showed her how to use the brush. She's a natural. Except that whenever I said "brush" she would run it through her hair. (Not a hair brush, silly girl. A paint brush.) She covered a few square feet of pavement with gentle brush strokes. And then she turned into Jackson Pollack.
Have I mentioned how much I love her little smock? Even if I have, it needs repeating.