The biggest thing is variation. Leg yields, circles, etc., will be boring quickly for your mare. Work on everything you can think of. Work with no reins, work over a ground pole, do a leg yield then a half circle and reverse, etc. Keep your mare interested in her work.
Work on your groundwork. Do some walk-canter or trot-canter transitions. Bring in a tarp and work on her crossing the tarp. Bring in a partner and a soccer ball and practice tossing it back and forth, to get her used to objects moving around her body.
There are endless possibilities, just be creative!