This is such an easy activity, with virtually no mess. It's perfect for hot days!
All you need:
bucket/bowl of water
a paintbrush or roller ( a nice, big house paint brush works great for this.)
Just let your child "paint" using the water onto a concrete sidewalk. The water will leave markings on the concrete. Letting your child "paint" a fence or outside wall is also a fun way to do this activity. It's great imaginative play. And go ahead and let them splash around in that water. It's hot outside. ;)