I do a lot of hill work and gallop sets on the conditioning trails, Jessica. I really like it, but I have some “playful” OTTBs that would rather be silly than do fitness, so they need ring work. Totally agree on hillwork though.
I incorporate Cavaletti work into every ride to work on footwork, hind end strength, and gaits. I use spiraling in/out on a 20M circle for some horses that need it, but not always do I use it. Totally good ideas though, Emilia!
If you have a stretch of land maybe sloping upwards u could try doing gallop sets. It really helps with stamina and pacing.
I love to do a lot of circle work, making the circle bigger and smaller as I go. I also do a lot of gait transitions and cavaletti/ gymnastics excersises as well.