Here's a good way to enjoy a hot summer day without a swimming pool. For this game, you'll need some spray bottles, or empty dish soap bottles with the cap on.
The player who is "It" gets the spray bottle and must tag the other players by squirting them. The person who is tagged first gets to take over the spray bottle and do the squirting.
Try playing this game as Freeze Tag. You must freeze in place when you are squirted. You can be unfrozen by having another player crawl between your legs. If "It" manages to freeze everyone, he or she gets to squirt again.
If you have a backyard pool, this is a great game to play in the shallow end. Waist-deep water makes it harder to get out of the way!