Get a toy! XD I just got a Jolly Ball for my horses and will hopefully be able to test it tomorrow. I love romping (safely!) about with my guys, it's just fun. Loki adores peppermints, it's just flat out fun to show him one, then make him give chase around the shade tree in the paddock
If she's kept in the stable during the day they also sell things like treat dispensers (or you could make one yourself) and licking devices for bored horses.