Here, Have A Free Night's Black Agents Adventure (It's 5 lines long)

The two easiest systems I ever saw for fun low-prep one-shot gaming are Marvel Superheroes FASERIP and Night's Black Agents--Marvel because the fights are so long, but hyper-kinetic, NBA because the players can't really do anything without talking a lot (thus doing a lot of the GMs' work for them) and because assassinations and heists are way easier to make up than the mysteries that drive NBA's parent game, Trail of Cthulhu. All you need is a building.

Here's everything I needed to run adventure that kept my Night's Black Agent players busy for four hours:

• It's Die Hard, except…
• There are more of them
• Your employer's got a tip the Gruber gang is coming in 6 hours or less 
• And your employers wants you to get into the hotel and get something out of the vault before the bad guys do
• And they won't tell you what's in the vault, just the box number
• And what's in the vault is, of course, horrible

Grab a skyscraper you've been to off Google maps, give the bad guys 2d6 health and 2d6 points to blow on whatever and you're ready to play.

And remember: dead civilians force stability tests.

My players' first move was to spend a bunch of points to create a cover as a celebrity chef "Like Gordon Ramsey, or just…as Gordon Ramsey."