So, here is my approach, in the context of a campaign. On my campaign, I tend to have a short list of NPC/Faction/Place and enjoy keeping the campaign on a shorter space rather than a whole multiverse.

So my technique would involve.

• Ask the players to give me a summary of latest session, that I'll crosscheck with my notes.

• Ask the player what they want to do, following these events. having reccuring NPC/Places/factions mean that I can improvise how these person react to the event (if they do). This will easily burn a hour.

• While all of that happen, I have time to think about how to relaunch the story, either there is an event which absolutely makes sense in the context The local mafia isn't happy that you dismounted their drug production lab, when you come home you find a miniature coffin with a bullet inside in front of your door or, even though it's a bad practice, I throw a "randomish encounter" A big etheral cloud forms over the magic equipement store, and you can see some ethereal creature leaving that cloud and ear screams of bypasser being attacked The latter adds a combat buying me an extra hour to find-out why this shop exploded.

• Then, I can let the player investigate these events, it may-not be the most complicated investigation I ran, and kinda linear, However, it's enough to keep going to the end of the session, and have new elements to develop for next time

For a one shot ?

In general, I organize them when they're ready, and I have a lot of one-shot scenario ready on my computer, alternative would be pulling a zero prep game.