The Zaratan.Zaratan, as you can see in the picture, is a giant tortoise with a full-sized tropical island on its back. Its stays asleep for thousands of years, and when it does wake up it mostly just drifts along with the currents so any inhabitants of the island would likely never know of its existence. What a great setting for a multitude of adventures! A "Lost Island" that floats around the ocean, and if you're lucky enough to find it you'll probably never realize its actually a giant monster unless something happens to make the giant tortoise submerge (and then you're boned). Or perhaps you could get swallowed by the Zaratan and if you don't die right away, there could be a whole civilization of shipwrecked pirates or sacrificial victims (from the cannibals populating the island on its back) living in the Zaratan's slowly-metabolizing guts.
So yeah, I guess I'm kind of cheating by picking a monster that really is a campaign in of itself, but there you go. Plus I like turtles.