Horseshoe Toss
Set up a horseshoe toss game for people to play outdoors. If you want to keep score hang a white board with the pen and eraser near the game so guests can write their name and scores or win/loss stats on the board. You can also make this into a championship if you'd like where one cowgirl plays until she loses and everyone's had a chance to play. Last player standing wins.
Squirt Gun Target Practice
Set up some plastic bottles or toys to use as targets. Fill squirt guns with water and challenge guests to hit the targets. The person who hits all the targets in the least amount of time wins. Or make the game a squirt-gun war where guests have to face on and see who is the fastest to draw their weapons. Give each person a holster and place the squirt guns in it. First to draw and squirt her opponent wins.
Stick Horse Relay Races
Purchase or make stick horses out of mops. Let kids "ride" at will or in set patterns. Set up a race area where cowgirls can have a relay and see who "rides" her horse the fastest.
Cowboy Boot Pinata
Get a cowboy boot-shaped pinata. Let the birthday girl be the first to take a couple swings then go in order according to age, starting with the youngest. As soon as the pinata is bust open it's time for the kids to grab all the candy or other contents.
Square Dancing
Teach everyone a basic dance or two and then turn on the music and let them practice their moves.