I've always wanted to run a "dungeon keeper"-esque one-shot where the players are the weakest kobolds of a tribe left behind when said tribe attacked a goblin hive.
Now, these weaklings see adventurers coming, and they must save the leader's room, the treasure chamber, the children and themselves. Fighting would be futile.
I'd map out the kobold lair, give them a timeframe and show them some traps they could built. From then on, just lean back and let them think...
My favorite one-shot I actually run was a christmas game. Basically, the players got plane-napped to the world of kuhrissmess. There an angry white dragon mother had retaliated on Santa Clause because he wouldn't give her three children a present.
She'd frozen Santa in a block of ice. Santa's elves had mounted the emergency sled and wanted the players to save him.
The players had full creative reign over the presents they wanted to bring - if any. On their way, frozen lakes, unstable bridges and yeti brought a little danger to it all.
It was cool. And now I'm ashamed that I don't remember the presents.
