Great mason jar idea - try to make this on your own! - you could really use any old jar - soak the label off with oxyclean and warm water first. You could do Christmas jars, too. white for snowman, green tree with sequins glued on as lights.
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.