I'm no teacher, but here are my answers:
You can say:It's said in North America sometimes people say 'We'll have coffee/lunch someday' to mean 'Goodbye'. My question is how we should reply? Should I say 'Goodbye'?
Yeah, will call you. Later!
(you must answer the proposition somehow, even though nothing is fixed yet).
You can answer 'ok' if you don't want to elaborate on the question. It is generally asked out of politeness, or meaning 'anything special at the office today?'. If you did you personal assistant, you usually answer 'no, no!'.When I watch a film the wife asked her husband 'How is your day'? But it seems the husband didn't tell her about his day, such as work. As far as I can identify, he only said something like OK. My questions are: 1. Does it mean the question is something like 'Hi'? So the listener does have to take it seriously? And 2. Is it a question only used among husband and wife or it can also be used among colleagues? 3. Is OK the standard reply?