First, vibe check. Let the players shoot the shit a little more than usual out of game. While they do this, you do a little last minute brainstorming or note-taking/reworking.

Second, if you let them drive the convo, they will usually give you some clues when they finally get bored and start asking each other to calm down and start the game. Things like “I want to find out what so and so has to say about the mission we just got from who’s his face!” Or “I want to kick that (minion of the bbeg)'s ass! Let’s get moving!”

This tells you what your players want. Now you have some focus on what you need to spitball.

Now it’s down to your improv skills. Yes-and helps a ton here. You ask what they do and it just works, or works with consequences. Ask them to roll some checks and if they roll high and it isn’t stupid they succed and do the thing or get the info.

If they roll low something bad but not lethal happens. Minions show up, NPCs laugh at them, etc etc.

If you panic, ask them to roll a check and figure out what is is for while they are rolling the die and adding the result. At some point it’s just art which you get good at with practice.

Good luck!

My favorite go to, one I’ve used twice in the same campaign and no one was the wiser, is to throw some ridiculous fight at the party out of nowhere, let them sweat it out for a round or two, and start dropping hints it isn’t what it seems.

I had them stumble across a black dragon in a cave as a lvl 1 party once. After scaring the shit out of them, for a round or two, someone “finally noticed” that the wings seemed to be made of tar covered cloth. Druid did a nature check and realized that’s not what a black dragon roar sounds like at all. Literally 5 kobolds in a dragon coat.

One time, I thought we had canceled but everyone pinged me about why I wasn’t logged in to roll20 yet (got my weeks mixed up). Luckily one other person did too, so I told the party I was going to puppet their character so they would level up too. I had that character betray the party by leading them to a trap. They defeated the player character (I used their actual character sheet to fight the party), for them to discover it was a doppelganger, and the trap was the diopleganger’s lair. they solved through a bunch of traps and random creatures from the diopleganger’s managerie of tortured -to-the-point-of-insanity minor monsters until they found the actual player character that (as they discovered) had been kidnapped the night before.

One other time l, over lockdowns, I had a friend miss a few months of sessions due to some serious and very depressing circumstances. He still wanted to continue once life had calmed down. We were doing an Avernus campaign, and I had been NPCing his character, but I told him to fast forward to his character to the current party level (about 6 levels) and not tell anyone he was going to rejoin the play sessions or log into roll20 until I gave him the go ahead. About 15 minutes in, the party is sailing down the river Styx when they see a damaged flying fortress crash landing, streaking by overhead. They hear a hellish scream and see a buck naked tiefling jumping out of the ship directly for their raft. At this point my friend logs into discord and yells “I WANT MY SHIT BACK YOU IMPOSTER BASTARD!”. combat began immediately whereupon he fought himself and regained all the loot the imposter had been carrying. The party had a hell of a good time that night, and he never did explain (in character) what hell actually happened to him.

I don’t schedule a game if I’m not going to be prepared

If I have nothing prepped, it’s goofy random sidequest night.

One of my favourite sessions ever started when the player the session was focused on called out due to work. With my plans derailed I had ninjas crash through the window, steal her famous recipe book, then run off and leap into the back of a van with her biggest rival at the wheel. It turned into a driving battle on the highway with characters leaping from vehicle to vehicle while a supervillain chef pelted them with capsaicin-laced muffins. It ended with an elderly PC duking it out with a morbidly obese NPC (neither of which had any points in fighting) in front of a police station. They traded single points of damage until the cops finished laughing and arrested them both.

A shopping trip can kill half a session if it’s been a while. Then maybe one of the shopkeepers has a problem that would be worth one of the nicer items in their shop if it were taken care of for them.

Get yourself Sly Flourish’s Lazy Dungeonmaster and The Return of the Lazy Dungeonmaster, first of all.

To answer the question, think of a strong start, maybe a combat to give me room to think between turns. Think of 10 secrets and clues. Grab a map from Dyson. 5 Room Dungeon it. And if you’re pressed for time, ChatGpT can be super handy pulling together tables and last minute prep.

Sexy goblin

I gave myself a vague outline of a plan, I’ll wing it.

My players stumble upon an abandoned version of one of my Dwarf Fortress cities.

I already know the whole layout backwards and forwards without needing to work with a map, and it has traps, epic architecture, purpose to every room, little random stuff scattered through, and I can usually come up with some kind of theme or ending to the dungeon for if they get that far into it. Easy peasy.