We have to do lunch sometime
We('ll) have to do lunch sometime,and Let's do lunch (sometime).
We must have lunch together sometime. (A vague statement that may lead to lunch plans.) Rachel: Nice to talk to you, Tom. We have to do lunch sometime. Tom: Yes, good to see you. I'll give you a ring. Tom: Can't talk to you now. Catch you later. Mary: We'll have to do lunch sometime. John: Good to see you, Tom. Tom: Right. Let's do lunch sometime. John: Good idea. I'll call you. Mary: Catch you later. Sue: Sure. Let's do lunch. Mary: Okay. Call me. Bye.