DIY Glitter Mason jar candles - Mix water with Elmer's glue and brush the inside of the mason jars. Add glitter of your choice to the inside of the jars, and roll/spin the jar around until the glitter coats the sides. Let dry and add a tea light!
cocoa powder (depending in how tan you want to be.) + Any moisturizer + an empty bottle + mix it all together. Good for super light girls. So is this like a real tan? I don't think so. But I guess if you wanna be tan for a special occasion it could work
Gummy worm ice cubes - infest drinks as they melt. Put the worms in an ice-cube tray, letting them stick out of the top and sprawl over the edges, and fill tray with water; Add cubes to a clear beverage, and watch as kids squirm with delight.
Glow sticks in the pool, great for night swimming! Not sure this would be the best idea for a big in ground pool (would probably take a couple hundred sticks), but would definitely be cool in a kid pool or an above ground pool!