I find GMs often have a very quick adventure. Something they can run at a moment's notice, almost regardless of system or genre. Sometimes there are limitations on that, but often they can be twisted or coopted to do what needs to be done in the event you need it.
Do you have an adventure like that? If so, how does it work?
Amusingly enough, my absolute favorite quick adventure is pretty set for the Super Hero genre, but you can make it work elsewhere. It's actually a book adventure and came in Palladium's "Heroes Unlimited" Revised Edition. The mission in question being: The Mall of Terror.
The Mall of Terror is a simple setup, a local shopping mall has some super villains taking it over. The super villains have hostages inside. They've made exorbitant demands of the city. And the PCs (the heroes) have to find a way to stop the villains and save the hostages.
What I love about the mission is it is endlessly repeatable because who the villains are changes the challenges in the mall and how the mall is set up to be defended against heroes.
What I also love about the mission - though maybe less so these days - is you can use it to quickly get people into the scenario because you can use whatever mall you want. Have a local mall? Use that. Now your players know what you are talking about when you point out which shop or which area. They can easily imagine where they are, and you can run with that.
This also makes it a good introductory mission for new players because since you are using a landmark from the area, they don't have to strain as much imagining things. And since they know the area, they can also be creative in applying things. I more often get an attempt at creative solutions when the player - especially a new player - doesn't have to build what I'm describing but can just recall from memory.
What about you? What is yours? How does it work?