What objects should I include in a model of the planet Earth in order to represent the prince's journey in Saint-Exupery's The Little Prince?
If you are reconstructing the planet Earth for the little prince as he experienced it, then what you should do is go back over the story and look at the prince's experiences on Earth as he relays them.
For example, we learn in the middle of the story that the prince landed in the Sahara Desert when he first arrived on Earth. We know this because when he first meets the snake, the snake very obligingly explains that they are in Africa and that there are no people in the desert, as we see in the lines, "This is the Earth; this is Africa ... This is the desert. There are no people in the desert" (Ch. 16). We can deduce that the desert he has landed in is the Sahara because later the pilot understands that the reason he met the prince in the desert is because the prince had returned to the desert in order to return home. Therefore, at least a few objects that should be included in your model of Earth is the continent of Africa, the Sahara Desert, and a yellow desert snake.
In subsequent chapters, we learn that he crosses the desert, converses with a desert flower, crosses a mountain range, as well as many other things. Therefore, all of those can also be included in your model of the Earth as the prince discovered it.