Just an anecdotal bit of support for PtheP's ankle suggestions:
I have weak ankles, too. I was in Ace bandages or an air cast more than I was out of them between jr. high school and grad school.
But after a particularly nasty sprain/stress fracture, I was told by a doctor to start doing the "standing on one leg" exercise in PP's link whenever I happened to think of it, including a bit of wobbling to challenge my balance and work the ankle harder. I often do my one-legged wobble when talking on the phone or washing dishes, or even when standing in line at the bank. It's totally free, ridiculously easy, and it WORKS!
I can't remember the last time I sprained my ankle. It has been over a decade. (20 years, even? Not sure...) That's serious progress! Plus it improves one's balance.
I have long since dropped the other exercises suggested to me, I'm afraid.
But the "one-legged wobble" keeps my ankles safe and strong.
