Here are some games to play out in the yard on a hot day. Unfortunately, the last ten years of my kids' lives have been during a drought so I couldn't really do this with them. Or if I did, they were too young to remember. But who knows, they might someday move to a place with 100,00 lakes, right?
Here's one my mother did with us when we were kids:
Everyone gets a bucketful of water and a small ice cream container and takes it to a corner of the yard. Everyone should be equally far away from each other, and maybe ten steps apart.
When the eldest yells "GO!" everyone scoops some water into the ice cream container and throw it at someone. If you let go of the ice cream container you'll get soaked trying to retrieve it. You also don't want to get too far away from your base or you'll get soaked on the way back. You also don't want to run out of water before everyone else because then you'll be a target without any protection. Of course there's a danger you'll slip on the wet grass and hurt yourself, but we never did.
Another thing we did was run through the sprinkler, and we actually did hurt ourselves doing that. I once landed hard on my back and winded myself, but I didn't care.
A gentler, safer game is to fill an ordinary balloon half full of water and pass it around in a circle. The idea is, you want to pass it quickly, but you don't want it to burst on you. If everyone's too good at this, then start two going around the circle in opposite directions. Or pass them across and hope they don't collide.