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.
Take a 2 liter soda bottle, poke holes in it. Attach to a garden hose. Toss over a tree branch and let hang for a kids water sprinkler. You can loop it over a tree branch and let kids bat it around for more watery fun!
Blooming Backdrop- Just tape vertical rows of long-stemmed varieties to a wall (use blooms less prone to wilting, like peonies, carnations, orchids, and lilies) and let the strips show -- or hide them under petals and leaves.