You can decode "Let's have coffee" in many ways.
Let's have coffee. Let me help you.
Let's have coffee. Let's do something together.
Let's have coffee. Let me sell something to you.
Let's have coffee. Bye now, hope never to see you.
You'll save a lot of time simply by knowing the difference when someone asks you.
And the next time you tell someone "Let's have coffee," I guess it's fair to tell them what's on your mind too :)
