Quite possibly the simplest exercise ever! Splat poles! They are called this, because they are just put splat on the floor without worrying too much about distance or position.
These are great to use as part of the warm up, and very easy to ride over. Although ideally ridden at a trot, they could if needed, be introduced to the horse at walk. Picking up a nice, balanced trot, the rider should keep the contact light and themselves balanced and simply just aim for the centre of the poles while keeping the rhythm of the trot the same. The odd angles and distances of the poles will encourage the horse to lower his head to take a look at the poles and therefore lengthen across the neck and back.