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!
These pools by Master Pools Builders will grab your attention. Imagine sitting in a dry sunken island-lounge area in the pool. See these and more amazing pool features. Article in Luxury Pools Magazine, Spring 2010