It's easy, it's free (unless you need to snag a pool, which can be as cheap as a few dollars), and everybody loves it! See? Look at those happy faces. These kiddos will spend hours playing in the pool out in the yard. Don't forget the sunscreen, and remember to supervise those youngsters!
Don't feel like lugging the pool out, then cleaning it back up before it kills your lawn? I hear you.
Consider bringing a bucket and some cups and pitchers out into your yard. Fill them with water, and you've got hours of fun. Kids love splashing in the water, even without a pool! Make sure to be safe though--kids can drown in even a couple inches of water in a bucket. Make sure you've keep an eye on them.
These neat cups have holes in the bottom for extra cool pouring!