DIY - Sharpie Coasters, ceramic tiles (free or very inexpensive at any home store center), rubbing alcohol added with a straw or eyedropper, (for the colors to mix) -- Great inexpensive gifts!(Diy Ideas For The Home)
If you have an inflatable pool, take it out and fill it with the comfiest blankets and pillows you own! It is the perfect activity for night time to look up at the stars. Great for date nights or just hanging with the kids.
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!
Starry Night inspired luminaries (perfect for parties)