It is easiest to ask your instructor as they know what level you are at, and what you are working on. But if you have been working on trotting, you can always practice the sitting trot without stirrups, and once you are comfortable here, and if you are working on posting, you can practice posting without stirrups. My trainer made me do this when I first started riding, and it has helped me immensely. My leg strength is better, and my balance has improved greatly as well.
If you have poles or cones you can also set yourself up a fun obstacle course that has you working on turning with leg, and maybe add some halts and starts throughout. This will help you and your horse! Really if you use your imagination you can come up with some great fun things to do with your horse, while helping your riding at the same time.