I agree with some of the posts that the others have put up...
Focus on low impact activities, I have knee problems too which limits what you can do, swimming is good, walking ok, bike riding is also good.
Another thing which you can work on at home is core muscles, sit ups, crunches, etc. but before you start to do these which can put a load through your back you should really get it checked out.
It sucks when injuries stop you from doing what you want to, im sorry and my fingers are crossed for you