Let’s see. When my horse was a green bean (9 years ago!)…
I love doing serpentines from quarter line to quarter line, or perhaps a traditional serpentine with a transition on the center line (say, ask at the quarter line, be prepared for the transition to occur at center line, and the movement to be picked up again and the next quarter line).
10-15ish meter circle at one end proceed to a slight shoulder for. Or, if you do the circle at the prior corner, say about 10-15 meters, take it into a brief leg yeild, maybe to the quarter line, straighten, and proceed down quarter line and halt before the end of the arena.
Trot polls are always fun. I myself (ex-dressage rider now H/J) use them a lot in my training as I’m not over fences every ride. I like to set a pole on the center line, and do a 20 meter circle over it. It’s always fun when you have a bit more coordination to set up a fan looking set up in the corner (so the poles all meet at one point, but are going in a circular fashion) and doing that. But, for now, you might use two trot poles instead of five 
Doing a 20 meter circle at one end, “spiraling in” to about 10-15 meters, then “spiraling out” (leg yielding out) back to 20 meters.
Take parts of a test an incorporate it into your ride. Enter at A, halt at X, turn right at C, 20 meter circle at B, then add like, a quarter line with a leg yield, with a canter transition at E, a 20 meter circle at A, working trot at A, long and low 20 meter at E or C or… you get the point!!! And pardon my letters, I haven’t ridden dressage in YEARS so I don’t know, they’re probably messed up a little 
What I do (because I do not “do dressage” anymore) in my flat work is look at the arena and how it’s set up with the jumps, and make a course, only no jumping. I alternate where I would hand gallop, where I would collect, where I’d do a flying change. I’ll add some transitions in there, ask for a leg yield, a shoulder in, put a serpentine in it, and then finish the “course”. It’s helpful for me to have like, a stop and go point. Otherwise I tend to give up before my horse does! But when I look around the arena and go “where can I strengthen my weaknesses?” it really helps. For my horse and I, it’s getting a bigger step at the canter, so, I alternate between a working canter, collected canter, and lengthened canter between fences (that we’re not jumping haha), maybe add a few flying and simple changes in there, along with some circles. Work on the expression of the movement, the quality of the step rather than the “frame”. That will all come together in time, but if your horse is being a bit lazy, or faking it, forget about what you look like and ride a forward, balanced, tempo where you can eventually get to the point of being on the bit. However, without the basics, you’re going to be fighting a losing battle!
Sorry this was a lot longer than I thought it would be. Hopefully it’s not all gibberish, and may be useful at some point 