Horseshoes
Horseshoes is probably the classic backyard game for adults. Its rules are easy, it's quick to set up and it hearkens back its origins of an old-fashioned America. For a game, you will need eight horseshoes and two stakes. According to Party 411, the length between stakes should be 37 feet (which should fit in the typical suburban backyard). Teams play to 40 points with a point coming when a horseshoe lands within 6 inches of a stake and three points for a ringer. Teams can have up to four players each, each taking a turn to toss.
Chipping or Putting Contests
If you have some golfers in attendance at a party, you may try a chipping or a putting contest. For a chipping contest, it's best to either have a net to chip into or another target to shoot at. Set the target at various distances around your yard and split your party into teams. Distance from the target can be worth the points you determine. Depending on the type of grass and the makeup of a yard, you could also hold a putting contest. Place a target where the hole would be and putt away.
Bocce Ball
Another adult game that tests your aiming and tossing skill, up to eight players can play Bocce ball (two teams of four). The setup for a Bocce game is easy. It is playable on any type of surface. A regulation court is 76 feet, but you may need to shorten that to adjust to the size of your yard. Bocce ball begins by tossing a small ball, called a pallino, on the far end of the court. Then teams attempt to throw their larger Bocce balls as close as they can to the pallino. A point goes to a team for each ball it has closer to the pallino than its opponent. Winning totals vary, either 13 or 16 points.