Arm curls can be done anywhere with almost any injury. Sit down on the couch and R.I.C.E. (Rest, Ice, Compress, Elevate) the broken foot like the doctor recommends and grab some weights. Just raise and lower the arms. For a little diversity try different weights or angles. Work the biceps and triceps and even try for the shoulders. Between this and the crutches those arms will be stronger than ever.