On a tray, place a pile of baking soda. Mix vinegar + a few drops of food coloring in a cup. Give the kids an eyedropper. Have them fill it with vinegar, squeeze onto the baking soda and watch mini volcanoes erupt!
I have personally made this recipe dozens of times and even if you just keep the Playdough in a Ziploc bag, it lasts for forever! There is no reason to buy the "real" stuff. - 1 cup flour - 1 cup water - 2 teaspoons cream of tartar - 1/3 cup salt - 1 tablespoon vegetable oil - Food coloring (Tip from me: To make for more even coloring, add the food coloring to the water before you add the water to the rest of the ingredients).