Obstacles. Put out obstacles in the arena. Anything you can around the barn. We have a mattress, cones, poles, frogs that make noise when you go past them, tarps, hay bale with a cow's head, mailbox, pvc 'cave' with 'finger's that the horse goes through and the fingers which are made out of a foam material touch the horses side (interestingly this is the obstacle that freaks horses out way more than walking over the mattress).
It keeps the horse engaged and its something to keep the boredom away. Just make sure whatever you use won't injure the horse if they step over/on/around it (things that stick out with rough/sharp edges).
Unless it weighs a ton... it's just a horse. Draft horse motto.