My lesson students love to play the "mummy game" I stand in the center holding the end of a roll of toilet paper and the goal is to control their horse in a circle around me and wrap me up without breaking the tp, can be pretty challenging :) You can do the same thing with a jump standard and a lunge line if you don't want to be mummified lol!
Egg & Spoon is always fun, if you don't want the mess golf balls work well too.
Water races are fun when it's warm. 2 barrels, 2 - 5 gallon buckets and some cups. Set a time limit and see how much water you can transfer from full bucket to empty bucket.
Suitcase races - go to goodwill or a thrift shop and buy some horrificly tacky clothes and put them in boxes/suitcases...race down, dismount, put on your new "wardrobe" and race back.
The Halter Race - like musical chairs. Everyone brings a halter, start with one less. Toss around the ring or riding area, when music stops dismount and get a halter. Continue til one person is left.
Hope that helps, have fun!
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