Transitions, grid work, poles, raised pole exercises, jumping on a circle, 10m circles
Try working out the arena sometimes, poles, teaching something new, more advanced dressage movements, jumping exercise
Poles are fantastic as they are not excesively diffucult and the horse is forced to pay attention to you as well as to what is happening beneath their feet.
Circles, stopping, turning their heads different directions
Lots of different exercises in quick succession. If you would normally turn right after a certain exercise, turn left. If he knows what you're going to do he will lose concentration. Keep him guessing!
Circles and stops! At any gate just out of no where. They start being a lot more attentive
I would say figure 8. But make sure he is bending in both directions and you are using you leg aids.