These are the constrains:
It has to be a short action adventure. No convoluted plot or excess of named NPC's.
The PC's will be badasses who will be able to face any opposition thrown at them. The adventure is meant to be a challenge, but the players will be able to decide when and how to fail (if ever).
It has to have a climatic showdown (of any kind).
It has to have a clear goal and the players must get a McGuffin out of it (which can be either the goal or a reward for reaching the goal).
If you propose more than one adventure, you can link them through NPC's, themes or concepts. There is no need to worry about anachronisms, genre, time period, style or "realism". Any action genre will do. (Think Sucker Punch.)
This is the format:
[Twist] (this is optional)
Quest: Follow the White Rabbit (and capture him).
Scenario: The PC's are trapped in a weird and surreal land, where all the creatures and entities seem to come from classical games and nursery rhymes.
Twist: Native people from this land cannot die.
Boss-Fight: The Red Queen of Hearts and her feral pet Were-Rabbit.
McGuffin: The White Rabbit's magical clock
Quest: Rescue the Princess and bring her back to the King. (Preferably alive.)
Scenario: A huge castle on top of a mountain, where dark cultists are sieged by a horde of wolf-riding orcs.
Twist: The Princess is not a prisoner. She is the cult's head priestess. (The King was aware of that.)
Boss-Fight: The cult's summoned Demon Lord.
McGuffin: The Princess' circlet (which is the Key to a heavenly Gate).
Quest: Steal the Duke's Golden Heart.
Scenario: The clockwork city of Mechanisburg and the Duke's gargantuan zepelin/palace.
Twist: It is because of the Duke's firm hand as a dictator that the Empire has endured such a long period of peace. The aristocrats that contracted the PC's to do the job want war.
Boss-Fight: The Duke's Right Hand (a huge steam-powered war construct), either on the Duke's command or going rogue.
McGuffin: The Duke's Golden Heart (a beautiful female shaped construct with the soul of the Duke's beloved daughter).