So I've been wondering:
When people say "good morning," are they wishing that you will have a good morning, or are they telling you that it is already a good morning?
In support of the latter, I notice that people tend to say "good morning" more on days like today; sunny springy weekends. If that's the case, then an appropriate response would be "yes, it sure is."
But people don't say that. They say "good morning" right back. If the greeting is a statement of fact, then that's a bit odd. It would be like if I told you "Greece is the world's largest (per capita) consumer of cheese," and you responded with "Greece is the world's largest (per capita) consumer of cheese."
It must be more about well-wishing, then. That's a lot of pressure; morning only lasts a few hours, and it's tough to raise it up to a standard of goodness just because a stranger on the street told you to.
Maybe the most appropriate response to "good morning," then, is "screw off old man, I'll do what I want with my morning."