Ride squares. Pick up an energetic trot, ride straight, apply a balancing half halt and make a 90 degree turn. (or as much as your horse can do without becoming unbalanced) Repeat until you make a square.
If squares are too much, ride octagons instead of circles. Insert 8 distinct points and half halt before each one before changing his line of travel to complete that part of the Octagon.
Ride around the ring but stay 2 tracks off the rail. This takes both arms and both legs, really tests your power steering since horses like to be on the rail. This one's great for getting their attention.
For canter work, play trot-canter-trot. Sounds like you already have a good working trot going. Pick specifice places in the ring, preferably corners and ask for the canter. If he speeds up instead, come back to a balanced trot before asking again. Once you get the canter, let him canter for as long as he can do it in a balanced fashion. If he breaks, speeds up or flattens out, come back to a good working trot. He may get fussy and try to break into the canter again, but stay with the trot until YOU have the trot you want. Only then, pick another corner and canter again. At first it might help to ask for canter in the same spot until he understands the canter cue. After that, vary the places where you ask or he will quickly begin to anticipate and try to canter whenever you hit that spot.
I've got about a zillion more since I ride alone and get bored easily. I'll post more later.