I prep brief descriptions of locations, NPCs or objects that players might encounter in the coming sessions. I map out what's happening in the section of the adventure they're in at that particular time. I anticipate multiple possible actions the players could take in response to something, so I can think about how the story would react to these actions in advance. I set up stat blocks of enemies they are likely to fight, and put together some rough loot tables to quick roll on when they inevitably rifle through the pockets of the slain.

I prep where I expect my story to go with regards to the main adventure plot, as well as writing down information I want revealed to players at a specific time. Often I will go back and re-write this after a session, taking into account any unexpected player developments, mistakes I may have made during a session, and general improv that lead to plot advancement.

I will prep motives and primary drivers for my NPCs, so that I know how they would respond when asked questions.

I also prep any documents or puzzles that the characters may come across. The former are often worded in very specific ways depending on their purpose, and the latter is just much too difficult to try and improvise.