Like a lot of people here, I initally had the "what the hell" reaction.
And I thought about it, for several hours and up to this morning. Then the whole thing grew on me, it just clicked! I wrote some scattered notes about the ending, which I'd like to share with you:
So, what did I gather from all of this? I think that the secret (in a way) is that the Monkey Island saga are Guybrush's adventures, as told by him to his son (kind of like how it happens on the TV show How I Met Your Mother, which means: exaggerating them and changing their details when he feels like doing so) .
This reasoning allows for every single game - even the monkey head becoming a robot - to be canon (if we really want to talk about canon anyway). The games are just a visualization of Guybrush's adventures, but in the way he tells them to his son: e.g., EFMI happened, but some details might not be necessarily real.
The actual secret that Guybrush found that day might not be important (and in fact it might have been fairly disappointing to him, a thing which brought him out of the "loop" of the obsession over this fabled thing). But the bottomline is that this is a father teaching a valuable lesson to his son: if you build up exaggerate expectations over something, it will NEVER live up the hype and you might risk obsessing over it endlessly.
Of course there's still some room for interpretation, IMHO; the very ending might even be literal, in the sense that Monkey Island stories might be:
In part tales of Guybrush in his very early youth, lived in the times in which the very concept of pirating was starting to go out of fashion and "modernity" was starting to sink in;
In part outright fiction which he made up for his son while working (after his youth) as a flooring inspector for a pirate themed amusement park.
This reasoning might even justify the fact that Elaine found the map to a treasure, just to help him relive those same very old adventures.
Finally, the final decision about what the secret is up to the player: what was the secret? Gold and jewels? Friendship? It's better not to know it? Anyway we decide, this choice will directly influence the post-credit scene, which might actually might constitute the only things which we are sure that really happened in that world.
Again, we are the one to decide what really happens! We can even re-open the "employees only" door with Stan's keys and take the backwards path for a different ending with a different post-credit scene (Guybrush and Elaine sailing on a ship), which might just mean: Guybrush didn't want to believe/know the secret, so he just bailed on it, with the consequence that the MI stories are real and not some kind of metaphore.
I think this is a clever way to try and make everyone happy.