You can do serpentine's, circles, figure 8's, teach him turn on the haunches and forehand, you can also do some desensitizing like walk through some hanging noodles, over a box, work on being able to open and close a gate while mounted without having to let go of the gait. Teach him some tricks. walk trot transitions, trot canter transitions walk canter transitions. Flexing. Shoulder in/out. sidepassing. Backing up through poles that are in zigzag motion. Hope some of these give you some ideas.
The wind of heaven is that which blows between a horse's ears. ~Arabian Proverb