First, ouch!!! I made a stupid move on Saturday that ended up with an icepack on my right knee--my "good" knee!--so I can relate. It depends on how you've injured it, but most of my knee injuries respond best to good old RICE.

Second, tree pose is good for working on balance--and you can do it pretty much anytime you find yourself standing somewhere. I sometimes do it behind the podium in classes when I'm just lecturing.

The book Strong Women, Strong Bones (or words to that effect) by Miriam Nelson has some good balance exercises in it. Be sure you're getting enough calcium, too, so you're not endangering your bones.