Bowls made of confetti! Great recycling art project--save all those leftover paper scraps (put in small bowls at each table). Collect a bunch and have the kids cut them up; Then make bowls made from confetti.
Floating Flower Tip - Most flowers float pretty well on their own until they start to pool with water and slowly sink. For a longer lasting centerpiece, use bubble wrap to make little skirts for your flowers.