What workout routines would you suggest for someone with severe knee injuries?

[willyqpublic]

I have some damage to my ACL that required surgery, and while I used to be an active runner, I find myself unable to do so anymore. I've started to get rather fat due to my lack of cardiovascular activity. I need to find cardio that I can do that doesn't involve swimming, as a pool is not easily available to me. Thanks in advance.