Game of tag on a scorching hot day, squishy sponge balls are summer's alternative to snowballs. Made from dollar store kitchen sponges that are cut up and tied together, they last longer than water balloons and are softer and splashier than regular balls.
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.