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.
*Colored ice balls* Fill balloons with water and add food coloring (be careful not to let the food coloring escape!), once frozen cut the balloons off and voila! Look like giant marbles :) best winter decor ever. also, use them in the summer time