Not quite a medicine ball, more like one of these:
I've seen people do lunges with the forward foot on one of these, or other exercises while standing on them. I guess the idea is that since you have to keep your balance while doing whatever it is, you're strengthening a lot of ancillary muscles at the same time, ultimately improving your balance. Are these things useful at all? Anyone have experience with them?
Try balancing on one leg and throwing kicks with the other.Keep one foot on a step or something similar,and do not let the kicking leg touch the ground.After a while you can work on combos and more reps.Fun!