I love the murder mystery idea! Unfortunately, with 50 students per class, I don't know if I could pull that off.
One game that I've used with adult students is "I've never..." Basically, students take turns coming up with true statements beginning with the phrase "I've never" and they get points based on the number of students who HAVE done that thing. So the idea is to come up with something you haven't done that most people have done. For example, a good one for me (before I came to China, anyways) was "I've never gone skiing."
It's also a nice way to practice present perfect.
I'm not sure what to do with my 50 student classes. I think I'll just cop out and bring the guitar.
Good luck!
Dave