I do poles, transitions,circles in the corner of the ring and side passing
Transitions, transitions, transitions. They work great to get the horse in front of the leg, both in between gaits and within them.
As for suppleness, lateral movements, leg yields and different kind of serpentines and bending, changing the bending lines and serpentines constantly will keep the horse on its toes and more focused on you, just remember to not surprise them! I’ve also found that on some horses counter canter works great on getting them more supple.