Balloon Rockets - I've done this in several science camps I've conducted. Kids have fun with it. But, when the balloon deflates it may fall off the straw because it's surface area changes - another good thing to show and explain to kids.
Science Games: Awesome idea from Earth Mama's World: Food Chain game out of Styrofoam cups! Just glue images on to your cups and write the name of the organism on the rim. Mix them up, have kids put them in order, and stack them!