Thanks for the encouragement, y'all. I really needed it today!
This horse is a terrific instructor. Not because he's easy to ride, but because he has a virtually bottomless bag of BS to pull tricks out of for his rider. He's a safe mount - none of his tricks are things like rearing, bolting, bucking, biting, or kicking. They're more along the lines of taking initiative and trying to trot without having been asked to trot, or battling for control over the reins, or cutting corners, etc.
I feel like he works creatively to match his tricks to the demands of the rider. That is, when I wasn't really demanding anything of him other than to walk forward and stop on request, he was totally compliant and stopped on a dime...but he was kind of a butt on the ground - pushy, leaning on me when I picked his hoof, butting me with his head. Now he behaves in a princely manner on the ground, and saves his little tricks for under-saddle.
Usually, he just picks one trick, seemingly at random, per ride. It's great because it's giving me lots of practice at dealing with Horse Nonsense. They key, as my trainer has told me 1,000 times, is to figure out what he's challenging me with, and then stay one step ahead of him at all times (and she provides great suggestions for ways to do that).
This was the first time he's brought multiple tricks out of the bag...at one point, he had three of them going simultaneously. Given how we know that he ramps up his challenges to meet the skill of the rider (or at least, the level of demand the rider is placing on him) I am telling myself this must actually be a compliment.
And it really, really helps to hear that people who are much more accomplished at this than I am still have bad days, so thanks very much for sharing!