What do you tell a casual acquaintance the day before surgery?
Good luck! - implies that there's luck involved
Hope everything goes well! - why wouldn't it?
I'm praying for you - sounds too dire
You'll be fine - oh, so you're a doctor now?
Well, if you need anything... - now is not the time for bullshit
I've run out of options. I'm going to stick with, "see you when you get back."