I use these exercise a lot, they are an absolute staple, both when producing horses and coaching riders of virtually all levels.
4 poles positioned at '12', '3', '6', and '9' on a 20m circle or slightly larger if your arena allows. It's very simple, you ride around the circle over the middle of the poles, generally in trot or canter. It's great for testing how regular and consistent your horse's paces are and working to improve the regularity of paces, it tests how accurate you are as a rider and makes you really think about control, it encourages horses to settle because it's very predictable, and the poles help to create more hind-end engagement whilst inside bend is maitained on the circle. More advanced horses and riders can raise the poles slightly, and advanced combinations can replace the poles with fences (but don't go too mad with the height, steering is required for this exercise!).
I am a dressage trainer and general equestrian coach in Surrey, Sussex, and Berkshire. I teach dressage lessons, and hold a range of riding and equestrian clinics around the UK, and use my blog to share horse training tips, advice, and resources.