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!
Shell wind chime, a great diy project after a day of collecting shells at the beach! Each strand you do can have shells from different places you have travelled to. You can create small and longer strands depending on where they will hang.