Ankles are more important than most people think for good runners - the ankles keep you balanced, and a balanced runner is a fast runner.
Achilles routine: Strengthens calves, ankles, and lower legs. Found here.
Alphabet: Standing barefoot on one leg, write the alphabet in the air with the other leg (by moving your ankle, not the whole leg. These can also be done while sitting down (in study hall, cafeteria, etc.)
Strengthening exercises: Runner's World video found here.
Hill workouts: By springing up-hill, a runner exercises not only explosive power but also flexibility and a good economic running style. Find a hill with a raise of 1/3 or so (19 degrees). Warm-up for about 15 minutes, then start springing up the hill with a bounding style and slower forward progress. Keep the body relaxed and the head tall; don't look at the ground. Try bounding up the hill for 15-40 seconds at a time (or 100-300 meters), then jog back down and give yourself a full recovery between reps (you may jog for recovery).
Strengthening exercise: Active.com article on various exercises - theraband suggested.
Ankle rotations: Simple stretching exercise; while standing, balance on one leg with only the toe of the other leg touching the ground directly beneath you. Then "roll" the ankle in small circles, with your toe maintaining contact with the ground.
All of the above exercises, and:
Ice: ice the location