Sorry to hear about the knee; I know how fucked up those injuries can be. I over extended my LCL a few weeks ago and while it's nothing serious, I still feel it during sparring. There are plenty of sports you can do with a bum knee. Paint ball seems like a fun game, but not very useful in any way.
Anyway, I train BJJ (gi and no-gi) 4-5 days a week. I used to do MMA, but it's the ground game that I always loved most.