A very well detailed Triangle Glyph Tattoo. You can see that the pink flower is fully colored within the glyph triangle symbol as it slowly branches out into the less colored parts of the flower as the colors are only concentrated in the center.
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.