If someone else is around see if there'd be some willingness to do a few games together. A favourite of mine is 4 barrels, two per rider, one at each end. Find six nice round stones about the size of a golf ball and place 3 on each nearest barrel. The game is to compete with each other by picking up a stone, racing to the other barrel and setting it down. If it falls off, the rider has to dismount, pick it up, remount then place on barrel then go for the second stone. The horses often perk up on the second run as they figure out how this works.