To me, "to do lunch" seems to have a connotation of eating lunch with a secondary purpose in mind - "do lunch with a colleague/client" or "do lunch with a friend" - as if "lunch" was on your to-do list, for some reason, and you wanted to cross it off. "To have lunch" sounds quite neutral to me, on the other hand. If a friend said "we should do lunch" to me, it sounds like a future invitation to eat together; if a friend said "we should have lunch", it sounds as though it's reached lunchtime and we should both eat, and doesn't imply "together". But other people may think differently.
@[tex]{{\underline{\blue{theanuragkumar}}}}[/tex]_