If I know I have a short session, I would grab one of two old games: either Ghostbusters or Paranoia. Published by West End Games in the ’80s, character creation is quick and easy, the rules are minimal (but enough to allow a GM to adjudicate just about any situation), and they’re just fun games.
I can practically run either of these games off the top of my head. They require minimum prep and minimum props. I’ve run Paranoia games where the players got so into it, they session essentially ran itself and I did nothing except make a few rulings for two-plus hours.
Of course, they’re both a little difficult to find these days, so if you don’t have access to my games library, I would recommend grabbing a copy of the excellent $5 RPG: Fate Accelerated. It’s not as easy for players used to a more GM-control-centric type of game (FAE is much more narrative based and if players aren’t comfortable having a level of narrative control, it can be an awkward game), but it’s quick to come up with a character and a small, $5 book and some dice is all you need. You don’t even need Fate Dice; you can use D6s, you just have to keep in mind conversions (1-2 = -, 3-4 = blank, 5-6 = +). In fact, there’s no reason you couldn’t run a rockin’ Ghostbusters or Paranoia game using FAE.