many canter / walk transitions , in canter 15m circle and make the circle smaller and smaller in a helix form and then bigger and bigger again in the helix form.
poles on the circle line,
Many in and out jumps (in a row)
Try 3-6 poles on the ground with 3 meters in between, or 5-6 poles on a circle with 3 meters in between (in the middle of the pole) try to keep your horse in a steady pace and when you succeed to do so, you can increase the distance to 3,5 meters, asking him to stretch his strides more. This exercise has to be performed with easyness and without stress. Hope its helps!