Set a cavaletties (small jump not bigger than 50cm) and set them up like cater poles. If you do that and set it up as a bounce that will also help.
Lots of trot to canter transitions. When asking for the canter, turn into a corner, give a little half check with the reins and immdediately ask for the canter. If done properly the head will drop in a bit and the horse should jump a bit into the canter and therefore be a more bouncy collected canter. Then go back to trot and try it again. The horse will learn to jump into the canter and collect instead of stretching out
Try working with with poles!