Ground work training isn't only about lunging or something active, it also isn't just practicing an action (like practicing to stand at the tie rail or cross ties, standing for picking out feet, standing at a mounting block or fence quietly, etc.) - note how these things are doing something, an outsider can see something being worked on. But there's something that I see getting forgotten a lot because it looks like nothing is being done. I even have clients that get upset when I "waste" a session on "doing nothing". But, to me, THAT'S THE POINT.
We're constantly asking our horses to do this or that or the other thing. I see many horses that have an issue with standing quietly. Why do you think that is? Could it be because we don't ever really ask them to "just do nothing" when we take them out? Now, I know we all hang out with them in their stall/pasture or even in turnout in the arena but that's not really what I'm talking about as your focus isn't on each other and the horse is likely eating or thinking about many other things (and so are you).
What I'm talking about is just standing/sitting quietly together while still being focused on the moment. The goal is for you to be focused on the horse and the horse you, but without expectations or goals - literally just doing nothing (think Zen). At 1st the horse will seek stimulation or look for something to do (and so will you), the horse will try to walk off, or play with something, maybe even nibble on you. They don't get in trouble here just gently tell them "no, that's not ok right now" (obviously not just by words but by gently moving the lead rope or pushing them gently away from you). Then the horse will start being quieter and maybe even start twitching - just allow this process! Sometimes the horse will now also start seeking a distraction. During the twitching process their state of mind changes (from alert and "on the go" to quiet and relaxed), which, I think, some horses try to fight at 1st. This step may take a while. Eventually the horse will become relaxed, with head lower, not seeking something to do. They may start yawning (some a lot, some just once or twice). If you stay in this process long enough and do it often - that they learn that this IS indeed what you want - they may even start to put their nose down in the sand/dirt and walk around (ok) and find just the right spot to lay down. Some just lay down and roll, some take a nap, and some never lay down but become visibly very relaxed. What is this process good for? It is teaching them that it's ok to relax around you and that you don't ALWAYS want something from them, that they don't always have to think "ok, what's next?". This is useful when they get nervous or you just need them to stand quietly with you.