Torgo is a giant turtle that slowly walks the shifting sea (big silt deposit). Ysall, once called the singing city, resides upon his back; a city of the uplifted animal folk (wild martians), Ysall now appears abandoned; but it is not; as the result of a long ago science-magic ritual, the city's residents are in cryogenic suspension. They cannot be woken- or their muderous and tyrannical god king, Ko-Dom, who lies locked in cursed slumber as a result of the rite, will awaken too. Since the rite, the city has been overrun with monsters, undead and beings summoned from the dreams and nightmares of the citizenry and their vile god. Recently a colony of Klaken (freaky bug people) have burrowed into the cold sleep chambers, compromising the wards therein; the seals have begun to crumble; how long will the dread Ko-Dom continue to slumber?
So I have a temple carven into the shell of Torgo, with a second level below it. The point of the adventure is to keep Ko-Dom* from waking up, although this will not be obvious immediately. I am thinking of starting with everyone in the balloon, during a sand storm, on the run from ninja-priests that worship a Roc as a sun god. They come out of the storm above Torgo. I think weird fungus bats or mars stirges will attack the balloon. So after that the PCs have to decide whether to crash in the city or outside** it, because the baloon is in need of repair. Torgo's shell is like the size of a small island and covered in forest.
The other idea I had was to begin the campaign directly after the crash of the baloon, after it has hurtled out of the sandstorm, with the characters unaware they're on a giant turtle. I can't decide. This makes the setting a mystery, with clues, like the fact that about once an hour or two Torgo takes a step and the whole world trembles.
The city/island on the back of a turtle or fish, aka an Aspidochelone, is a pretty old idea, but one I don't see used enough.
*spell it backwards
**and thus either catch up to and somehow climb onto Torgo, or wander the desert in search of back up adventure. I have no problem with them getting a glimpse of the fantastic city on the turtle's back and moving on, either.