Along the lines of what Tall_boys said, you can take an empty dish detergent bottle, and fill it with water. The kids can then use it to "paint" the sidewalk, by squirting water everywhere! A teacher of an art class that my son took did this, and all the kids loved it. And as she said, you do your "painting" before lunch, eat lunch, and come back out and your "canvas" is clean and dry and ready for more "painting"! smile