I suggest you check out Bill Hartman and Carson Boddicker’s self mobilization techniques. I have used both after a few sprains out on the trails as well as with my clients. Carson also offers a few good drills. If you want a more in depth but fun and functional approach, Gary Gray offers a few DVD series that cover both the foot and ankle which include various ways to ‘tweak’ the range you may be lacking. Good luck.
Active range of motion exercises like writing your name in the air with your feet helps with mobility. Also, standing at the edge of a step and letting alternate heels lower down helps with dorsiflexion range of motion. Once range of motion is restored I love utilizing the fitness band for strengthening the angle in all ranges including plantar flexion, dorsi flexion, inversion and eversion which helps greatly with ankle stability.
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.