I agree! Hill work is fun and really does help with that and you could also do a lot of pole work and some small cavalettis to really get the horse underneath himself and using himself. Even if you're not really a jumper you could do a grid with very small cavalettis or jumps. Hope this helps! Good luck!
Doing transition practice helps get them more on their hind end, using the muscle. Do walk/halt/walk, walk/trot/halt, etc transitions. It also helps keep the horse in tune with you. If you have access to hills to ride up and down, that helps. Collecting the canter helps if you horse is at that level, could also try cavaletties and jumping grids too on occasion.