I also agree with Danielle. Spelling the alphabet (with both feet!) is a good way to involve plantar and dorsifelexion, inversion and eversion, and tibiotalar rotation.
But keep in mind that limitations to ankle mobility can be significantly impacted by medical history–sprains, strains, fractures. So, I agree with LaRue that if there is any history of injury, check with your client’s physician of physical therapist to make certain that your workout plan is appropriate.