I saw this idea for chalk paint several places lately, but the place I visit the most is The Activity Mom, so I'll send you there as a reference. And check out Childhood 101 for more play ideas today!
It's pretty easy to make chalk paint. All it takes is water, corn starch and food coloring. In large bowls combine the water and corn starch to the desired thickness, adding the water to the cornstarch (as it is easier to mix this way) and then adding a lot of food coloring, colors of your choice. I made mine pretty much the consistency of milk, 2 parts water, 1 part cornstarch. Give the kids paint brushes, cars, balls, anything that they can bring the 'paint' to the driveway or sidewalk with. (This dries like chalk and washes off easily, so don't worry if it gets on the house, the car...)
We used tennis balls rolled in the paint and, since our driveway is basically a slope, they made neat patterns as they rolled after they bounced several times, as you can see in the picture above. Cutie Pie also got out his construction toys and painted them. He had a lot of fun taking them to the 'carwash' after!
This was a lot of fun using coordination to throw the balls (especially for Sweet Pea who is getting a good throwing arm) and led us to talking about colors and how they combine as well as why things roll and why the paint sprayed off the balls as they rolled (all science). It also made for very tired boys - they chased the balls and painted for an hour or more!