Outdoor tic tac toe using stone tiles and rocks. I would make a small garden for the kids and have this in it. Cute decor when the kids aren't playing with it. Great for when the mom is at work and the kids want to play
Take a 2 liter soda bottle, poke holes in it. Attach to a garden hose. Toss over a tree branch and let hang for a kids water sprinkler. You can loop it over a tree branch and let kids bat it around for more watery fun!
Dog Pond - Place a plastic kiddie pool in the ground. It'd be easy to clean and looks nicer than having it above ground. Big dogs can't chew it up or drag it around. This is obviously more fancy than a plastic kiddie pool underground, good idea thou