1 - At the trot, post 7 strides, sit 7 strides, 2-point 7 strides, repeat! Do this while keeping your horse at the same tempo....OR doing working trot (post), collected trot (sit), and extended trot (2-point). When I change my horse's tempo, I usually do 11 strides instead of 7. Just do an odd number and you'll stay on the correct diagonal. Of course, try it without stirrups eek!
2 - Tie your reins in a loose knot and drop them. Ride with your arms behind your back, out to the sides (airplaine), or straight forward (superman). Try it with your eyes closed. Try the above exercise like this too! Great way to learn balance...and have your horse listen to leg aids.
3 - More for fun...grab something from a different discipline! I used to ride this Dressage mare and one day for fun we did a little "fake" Reining. I'd canter her forward as fast as I could, then at C, slam on the brakes. Jump right back into the canter, then at A, slam on the brakes again. After a few times, she got the idea and really started to tuck her butt for our fake sliding stops. It also made it very clear to my body that I needed to stay with my horse during a halt otherwise I'd flop forward.