Plastic or canvas tarps placed over the lawn (or the garage floor) should set the scene well enough. But you can also decorate a paper tablecloth with painted handprints. Fill a garbage can with ice to chill drinks. And hand out colorful goggles as favors as soon as guests arrive; they’ll need them to shield themselves from the onslaught to come.
It can begin innocently—a Silly String war or a piñata filled with a slimy surprise, like cooked spaghetti. Then have a Jackson Pollock–style free-for-all with buckets and squeeze bottles filled with washable paint (tell guests to wear old clothes or bathing suits). To get really dirty, make an old-fashioned mud puddle. Fill a kiddie pool with potting soil and add water. When the party is over, hose off the guests and pass out clean towels.