The summer is the time for getting the kids outdoors!
Many kids are involved in outdoor sports, but there are also games that can be played that don't require an investment in equipment. There are tried and true ones like hide-and-seek, but how about something new?
If you're an adult looking to engage a group of kids, The 175 Best Camp Games: A Handbook for Leaders by Kathleen, Laura, and Mary Fraser [790.15 FRA] will get you started with fun games like "Slow Motion Tag." To encourage cooperation rather than competition there is Cooperative Games and Sports: Joyful Activities for Everyone by Terry Orlick [793 ORL].
Go Out and Play!: Favorite Outdoor Games from Kaboom! [J 796 GO]
Kids' Backyard Activities and Games: 25 Boredom-Busting Ideas for Tons of Outdoor Fun! [eBook]
McGillian, Jamie Kyle. Sidewalk Chalk: Outdoor Fun and Games. [J 796 MCG]
Rhatigan, Joe. Run, Jump, Hide, Slide, Splash: The 200 Best Outdoor Games Ever. [J 796 RHA]
If you just want to let your kids go outdoors without any structured play or games, that's a great idea, too! Angele J. Hanscom recommends it in Balanced and Barefoot: How Unrestricted Outdoor Play Makes for Strong, Confident, and Capable Children [306.481 HAN].