Slow Trot down hills and walk up hills.!if you can't do that I suggest cavalettis at a trot.!
Quick transitions between and within the gaits, elevated poles at the trot and canter , walking over raised cavalettis (more suppleness=better work), and lateral work, especially turns on the forehand (your horse will reach under himself with the hind legs more, again when he has the suppleness)
Longitudinal transitions in the canter, shortening and balancing and then going forward to a lengthened canter. Hills are a great supplement
Hills are the ideal tool. If you don't have access to hills, I would suggest transistions