Yoga on or off horseback for starters.
Mounted exercises, without stirrups at the walk, raise on leg at a time and draw a circle on the saddle with your knee in time with your horse's walk.
Kind of like the above exercise, but without circles, raise your knees one at a time so you're only balancing on your seat bone on that side...keep alternating.
Alternating toe touches. With left hand, tough right toe, etc. (tie reins in knot for this one)
Alternating (again) bring one whole leg back behind the girth, then the other. By whole leg I mean, think opening hip socket. From the hip down, you want to open that whole thing up and bring it back.
That's all I can think of for now. If there are any equi-yoga instructors in your area, it is money well spent.
You just have to see your distance...you don't have to like it.