Backyard Chalkboard - paint and mount an outdoor chalkboard on a boundary fence for a great activity for children. Less mess + the rain washes it away! Add some simple wooden shapes in a bucket so kids can trace around them & create masterpieces.
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
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. I might actually make this! You can loop it over a tree branch and let kids bat it around for more watery fun!