Shocker...I do similar to what MIEventer does ;)
Also doing primarily eventing, all my horses are cross trained except my jumper (who HATES trails....oh and we still do them but he hates them...) and the core theme to all of it is trot, trot and more trot. Trails are a great break from ring-work and keep the horses from getting bored, while introducing varying terrain that will help build the strength and conditioning of both bones and ligaments/tendons, key for any horse that is asked to perform with a level of athleticism. I also very deepness and types of footing, we ride in mud, slop, dry, hard packed, groomed, and hills and what not as well.
And more trot work. TONS of trot work. Better canter and better jumps and better turns and well, all out better RIDING comes from better trot work. By varying the pace and always maintaining a solid rhythm, the horses will build good muscle tone. From there the canter and jumping falls into place. Will work some at lead changes and picking up the lead I ask for (too many horses imo will pick up the "correct" lead out of habit rather than queue), and working on some lead changes, and also, almost as important as trot, adding in grid work OF!