My kids have had a blast making and finger painting our fence with paintable chalk.   Note: It washes right off concrete, but we have to scrub a bit on our fence to get it out of the grooves of the wood.

Ingredients:

• Cornstarch
• Water
• Food Colors (liquid is okay, but the gels are more vibrant)
• Dish soap

Mix about a cup of cornstarch to 2/3 cup of water.   It can be difficult to mix until all the cornstarch is dissolved.  Add a teaspoon of soap and food coloring.  Paint away!

If you want the paint to last for future paint sessions, microwave it for 30 seconds (you might have to add another ten or so).  You want the cornstarch to semi-gel.  The top of the paint will get a ring of harder looking stuff around it while still having liquid pooled in the center.   You will need to remix it to work any clumps out and the paint should have a gel-like consistency which has a longer shelf-life.