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
The Ketchup Experiment! Kid friendly site with lots of neat experiments! It has one that makes a rocket out of a film canister, which would be good for an grade space unit, but it has to many kid friendly expiriments. Great site for any science teacher.