What kinds of games do you have that can be played in the backyard? We have a bocce ball set, and the kids have expressed an occasional interest in a croquet set. Whatever sort of backyard game equipment you have, it could be just right for keeping the kids busy.
These can be simple things that take only one person, such as a hula hoop or a jump rope, or complex enough to keep a number of kids busy. I tend to favor things that take a few kids, in part because I have three, but also because a group of kids often plays longer… so long as arguments don’t derail things.