Ride a buck:
Sit on a dollar bill with half sticking out and have someone tell you to walk,trot,canter ecetera! Then whoever loses their dollar loses and whoever keeps it the longest wins!
Have barrels set up and on the first and 2nd barrels put buckers and use a flag in the first bucket and run around the outside of the barrels then drop it in the bucket on the 2nd barrel!
Have an egg on a spoon and go around the arena at a walk,trot,etc. and whoever keeps the egg the longest wins
Saddle bag race:
Have one person at the top of the arena holding saddle bags and another person running up the side and grabbing the saddle bags from the person, or you can have the person at the top with no saddle bags then have the 2nd person run up with the saddle bags and hand them to the person at the top. Then the person at the top runs back withthe saddle bags!