The plan for the lesson was to warm Tris up thoroughly over his back before it started, and then drop the stirrups and do a 30 minute lesson in shoulder-in and work on the canter a bit.
It mostly succeeded. The catch was a bit that I didn't get him really through and supple enough in my warm up, so we spent the first bit of the lesson working on leg yield at the walk and then trot to get him connected and responding. Then we switched over to shoulder-in, starting at the walk.
Tris and I have both ridden shoulder-in before; it's not new to us, but it's been a long time and we haven't really ever had good eyes on the ground when we've schooled it. Plus, I'm finding that most things are a bit of a re-learning, both because we were out for so long with his injury and because the new barn is much more dressage-focused than the old barn.
So, shoulder-in went reasonably well; Tris had a tendency to be overbent through his neck and not get his haunches through. I had a tendency to hunch to the inside and let my outside leg flop around uselessly. Correcting the shoulders through the outside aids and tapping with the crop helped straighten him out, and discipline corrected some of my postural problems.
It really got cooking in the trot, though: Tris had enough forward energy to really load his hind end, and when I got him lined up, BAM, I could feel the sizzle in his hind end as if I had unstuck a cork. In fact it became a bit too much at times and he got fizzy and rushed and disorganized at the end of the long side, so we then incorporated half-halts every stride or two to collect the trot more. It felt like every pass we made was better than the last and with great substantive improvement. I was feeling his body much better and able to catch when it was going out of alignment, I was sitting more deeply and through and straight, and we had some very nice passes. Best of all, I was really connecting through my core in the shoulder-in - feel the burn!
Tris was huffy and puffy from working so hard through his hind end and over his back in that shoulder-in, so we walked for quite a while, and then picked up some canter work. The goal here was not necessarily to school, but rather more to demonstrate where we are in the canter. Right canter met with approval: I am more straight and deep in the saddle, more even in my hands and seatbones. He's more supple and more flexible and on the outside aids.
Left canter was better than it has been, but still not nearly the right canter. So we did end up schooling that direction for a bit, as he flung his shoulders around on the circle and was not nearly as sharp to the aids in the transition. Trainer had us school counterflexion and back for a bit to get control of his shoulders, and once I got the feel of it, applying it correctly really unlocked some nice things in his left canter. Basically, before each "point" in a 20m circle, counterflex for one stride; then correct flex for the "point"; then counterflex the stride afterwards. It was a way really emphasizing my outside aids, and not allowing his shoulders to rule the day.
So it actually ended up being more like 45 minutes, all without stirrups. Hoorah and huzzah! I felt great, and especially loved schooling the canter without stirrups. It's not an all the time solution - in particular, I can't warm him up effectively without getting off his back and I can't do that for long enough without stirrups - but I loved pulling them for the second half and getting a workout at the same time we did more technical work.