One of Adventure Time's most distinctive aspects is its colorful setting, The Land of Ooo. The fantasy land is a new continental landmass on a post-apocalyptic Earth, centuries after a catastrophic nuclear war. Its dynamic landscape serves as the backdrop for the show's ten seasons, but it originally looked very different. The show's old series bible contains a look at an early map of Ooo, with a lot of key differences between the original draft and what ended up in the cartoon.
The biggest difference is how areas are segmented, as the original map was split up between the Candy Kingdom, Burning Lands, Red Rock Cliffs, Sandy Business, Verdant Plains, Ice Mountains and an unnamed forest. There's also the Cloud Kingdom and the Mystery Temple. These areas swallow up the whole map, leaving almost no room for the wide-open spaces that define the world's second iteration. The new map, on the other hand, tends to be more of a place of grassy plains, with a few smaller zones, like the Fire Kingdom and Ice Mountains, but even those changed.
The Fire Kingdom
The Fire Kingdom is moved south due to the alteration of Ooo's shape, shrinking in size to be a small portion of the overall continent, but it formerly took up more space. Its name also changes, going from the Burning Lands to the Fire Kingdom. Phoebe the Flame Princess isn't in the series bible at all, making it unclear at which point the creative team settled on a formal monarchy for it.
The Ice Kingdom
Like the Fire Kingdom, the Ice Kingdom shrinks in size. Due to the Ice King's presence in the original document, its overall shape is fairly similar to how its portrayed in the show. It's the Ice King's domain, without many inhabitants beyond flocks of penguins. The outline also mentions terrible monsters, but those don't quite manage to come to fruition. It's place on the continent also moves, shifting northward.
The Candy Kingdom
The Candy Kingdom is perhaps the most affected by Ooo's shift in geography, going from an entire section of the continent to a small city-state. It would later return to something closer to its original state during the "Elements" miniseries, which saw Ooo revert to four sections defined by its primal elements: fire, ice, candy and slime. It's possible that particular plot element was drawn from the original series bible, as Ooo's original zones bear an uncanny resemblance to their later incarnations during the miniseries -- with the exception of Slime's absence.
Everywhere Else
Ooo's overall shape changes drastically, going from a fairly standard shape to the rough outline of an eagle. The seven zones are divvied up into smaller sections of the map; although, not all of them make the cut. Candy, Ice and Fire become their respective kingdoms, and the Verdant Plains become the Grasslands. The untitled forest on the original map stays in roughly the same spot, but it shrinks to become the Dark Forest. Meanwhile, the Red Rock Cliffs are entirely gone; although, a Red Rock Pass makes an appearance in the show.
The Sandy Business doesn't have much of a presence in the show, but it is a fairly large section of the map. The Mystery Temple, on the other hand, doesn't make an appearance in the show at all, and it's not listed as one of the seven zones described in the document, so it was likely scrapped later in development. The overall shrinking of Ooo's key features is for the best though, since it leaves plenty of room for all the weird oddities that make Adventure Time a delight.