Do you do Jumpers? Hunters? Eq? Do you know any Dressage?
To keep it interesting for the horse, make sure they are focused on you and alert. Don't let them "die". You can do this with lots of transitions, bending and counter bending, circles, changes of direction, serpentines, figure eights, etcetera.
You can also purchase books that have various exercises, and you can work on perfecting those.
I rather like flatwork--it keeps me polished for when I jump.
EDIT-To keep it interesting for yourself, and to strengthen your riding, do no stirrup work, sitting trot, two-points, etcetera.
The sensitivity of the internet baffles me.