On an Earth-like planet, there is a massive plateau, standing more than 1km high, and the breadth of a continent in the middle of an otherwise ocean planet.
I know there are climactic issues to this, but instead of hand-waving it, the best answer would explain the processes that created this steep, high continental mass. High-level, and not magic and no humans.
On the continent (size of, say, Africa) is most biomes - desert, forest, lakes, rivers sourced by artesian springs/rain, and then cascade off the sides of the continent. There are plants and animals.
This is for a young adult novel, so I am not using the hard-science tag.