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!
Classic Summer Fun on a Budget: 20 Activities to Add to Your Summer Bucket List
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.