Serpentines, figure eights, variation in speeds. I also like to teach new things in the arena. Like, teach a half halt and practice/teach a flying lead change. Make obstacle courses, perfect backing, work rollbacks, do half-halts.
Teach him how to do different things like long trot, shorten his stride, how to collect himself, and how to keep a right headset. Work SOME circles, but pretty much avoid them if you have to be in an arena ALL the time.
Also, teach him to sidepass, how to pivot (while your mounted), and how to move his back end from leg pressure.
Change it up, do lots of things, and keep him busy.
Cheyenne, QH Paint Trail Mare. Happy Trails, Y'all! Twister, Appaloosa Barrel Horse. Turn 'n Burn! These 2 horses stole my Heart!