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!
DIY Dual Duty Perler Beads Coasters or Drink Covers Tutorial from Loppi.These coasters/drink covers can be made out of Perler or Hama Beads - so they are very cheap and easy to make. For lots more Perler Bead DIYs go here. The video is not in.