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.
Rainbow Bubble Snakes - Need: Half a waterbottle, rubberband, bubbles (dishsoap/water), food coloring, sock. Steps: Dip sock held by a rubber band on the water bottle in food coloring in dish soap and food coloring and blow. Have fun!