ROOMs: minimal units
1.
Think of the room as the minimal unit of prep. Faced with a blank page, just make a room. (Dungeon23 (Sean McCoy showed that making a room a day is a worthy creative exercise).
Preparing a room is the viable middle ground between two overwhelming extremes:
Planning an entire adventure (which can lead to forcing situations (rails));
Planning all possible player actions (which goes against role-playing as a non-finite game).
2.
A room doesn't have to be a room. The class is abstract, and we can determine its scale. It is possible to treat a journey of several days as a room.
3.
A room doesn't have to be a room. "Room" is a metaphor; it does not have to be a physical space.
Some might prefer the word scene, but I find that is inevitably linked to plot. Thinking spatially means thinking about situations.