To get it off the ground, I think that like you said "a scenic drive that an MPG competition breaks out in" is a good way to go with it. Have fun, keep MPG as a priority but not a strict competitive event. This means you can avoid classing, disputes, cheating, scoring, etc. Have fun, and add to it as you go, if it goes well then try to expand on it at the following event.
That's personally what I think would work, I have never attempted to set an event like this up though, so this is nothing more than my opinion.
__________________
|