Mini volcano - 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!
DIY Turn PVC pipes into a playground musical instrument. I wish this had the instructions with it, but I might be able to figure it out. This might even work as a service learning project for the playground