Maps

As Brian Aldiss wrote somewhere, sf writers work hard for a living. A mainstream author writes a novel about a set of characters whereas an sf author devises a planetary system with its orbits, ecologies, life forms, inhabitants, societies etc, then writes a novel about a set of characters living in or visiting that system.

Poul Anderson's The Game Of Empire begins with maps of the Patrician System and of different areas on the surfaces of its two colonized planets. See here. Studying these maps enhances enjoyment of the novel and shows us how much work was put into devising the system. There are places that the characters do not visit where further stories could be set.

Jerry Pournelle's King David's Spaceship begins with maps of the planets, Prince Samual's World and Makassar. These have to be, and are, different from Earth and from each other. Again, we see the scenes of the action and also other places that are mentioned in the background information.

In both cases, these planets are made to seem like real places but are encountered only in this one novel whereas many other planets are presented in earlier and later volumes of the same future history. That is a lot of creative work.