Check out this list of Creative and Fun Backyard Ideas and find some interesting things to do out there in your backyard. A lot of these would make for fun party games. Moreover they are so simple and cheap to make, and you could play them with your kids or family anytime.
Lawn Twister Game
Build an Outdoor Bowling Alley
Giant Outdoor Water Bed for Kids
DIY Outdoor Chalkboard on Fence
Bean Bag Ladder Toss