This is the whole study of equitation, but here are some exercises to change up your usual routine.
Play with your halts
Throw in a halt at any and every point in your regular workout. It can be a hard thing to remember if you don't practice them regularly. Put your cell phone in your pocket and set an alarm for every five minutes. Halt from the walk, trot, and canter both on straight and lateral lines. I guarantee you'll have discoveries about your transitions.
Play with the flexion on your laterals
We all have a tendency to sit in a comfort zone with our laterals when it comes to the degree of difficulty with the angle. Perhaps you have a regular habit of doing the half-pass from the letter F to X or only doing the leg-yield along the wall with the horse's head facing the wall. Change up your angles, whether it's increasing the angle in your usual lateral movements or changing them completely. Try your half-pass left from F to V and then turn it into a renvers along the wall followed by a leg-yielding right to the centre line or take a modest shoulder-in into a leg-yield by bringing in both shoulders. One of my favourites is to take two similar lateral movements with opposite bends and transition back and forth between them, such as a shoulder-in right along the wall and morph it into a renvers. With the shoulder in and body flexed slightly right I slowly change the bend left maintaining the haunches on the wall. I halt every now and then through the lateral and even make transitions. For example, in the shoulder-in I will trot, change bend for renvers and counter canter, change bend through the counter canter and transition to trot returning to my shoulder-in.
I find a lot of people get addicted to collected movement and forget to lengthen and extend. There is no collection without extension, no extension without collection. Sure, we can think of horses that do one better than the other, but you want to work both to get a supple and strong horse. Massage every stride to have a variety of paces within each gate. You want to prevent rushing (fast moving feet in short strides) and encourage calm ground coverage where all legs are expressing themselves in long strides. The walk and canter are where many go wrong. If you have no idea where to start, let your horse loose in the arena or get on a longe line to see what the default paces are at each gait. Take a longe whip and gently push your horse on and remark the differences. I'm a fan of what my dad would call a good "gallop-ad" at the end of a canter workout. Feel the wind in your hair and get a little uncomfortable.