Scenario Writing - The art of the one session scenario

I've run one session scenarios in the past, mostly on special occasions like holidays, birthdays and halloween.  Here are a few of the techniques I've used:

Holiday Theme - If you're running a game on a holiday then use it for your plot inspiration.  There are plenty of plot hooks buried in the characters, myths, folk lore and cultural references of the various holidays.  Such as:

• Good Santa / Bad Santa - Saint Nick has a split personality (how else could he make informed choices about whose Naughty or Nice?) this year his bad side has gotten the better of him with all the personality defects that go along with it.  The PCs are elves (or charity / department store santas) whose job is to keep him on the straight and narrow so he can deliver all those presents in time or maybe the world needs to do without this year.
   
• The Easter Bunny's been kidnapped - A bunch of do-gooding dentists and dieticians have teamed up to rid the world of the Easter Bunny.  Kids all over the world have suddenly lost their appetite for chocolate.  Willy Wonka is in despair and has hired your paranormal detective agency to find the culprits and set the world straight again.
   
• Halloween Jack - A notorious psychopath has escaped from a secure facility and the PCs are on the tube, returning from different fancy dress parties, when the power fails.  No-one knows each other but one of them is Halloween Jack, the cops are on his trail but all they know is he's disguised in a fancy dress costume.  Oh, did I forget to mention that you're all dressed in the same outfit with a Jack 'o' Lantern for a head.

Birthday Boy/Girl - Make the player whose birthday it is the center of the action.  If they're up for it you could turn them into the villain of the piece, depending on the player, you might want to let them know where you're taking the story and maybe even set up some cues for them to trigger some events.  It's nice if you can factor in a few "in jokes" or references for each of the other players, they'll really get a kick out of it.

One location - when the PCs are trapped in one location the focus turns to escape. Your job as DM is made easier as you just have to think of all the ways they're going to try to break out.  Having some of the more mundane "what-if's" pre-thunk means that you can concentrate on the more bizarre suggestions that players inevitably come up with.

• Internet / TV / Telephone / Mobiles - Do they exist in the genre and if so are they working, If not why not.  If you've let the PCs have access to a phone and diall 999 (that's the UK version of 911) will the cops come and if they do how do you get rid of them.  I really like using this as a tension mechanism, you can have them come out look around and leave as though nothing is wrong.  Or you can kill them in some bizarre or predictable way. Watch the looks on the players faces as they realise that they're responsible for luring them to their deaths.
   
• Outside - Can there an outside?, can they see it?, Can it see them?  If your players are trapped it won't be long before they start to think about what's happening in the outside world, you can play with their minds by making their prison a wrap around universe.  A player manages to escape a room through a door only to re-enter from the opposite side of the room or down the chimney.
   
• You are not alone - Don't leave it too long before you start hinting that they might be sharing their reality with someone or something else.  Maybe they put an object down moments ago and now it's moved or vanished entirely.  Perhaps it's a spirit hopping from one body to another, give each of your players a secret message containing instructions when they start playing the spirit.
  
• Horrible history or location - Take an event or location that your players know very well and twist it into an evil or sinister version.  You can be as wild as you want as long as you don't warp it beyond recognition otherwise the players may miss the gag.
  
• Player Secrets and Paranoia - If you want to ramp up the tension you can give each PC a piece of knowledge about one (or more) of the PCs at the beginning of the game. Later on that piece of info might be pertinent and the player must decide to act on the information.

Body Swap - Your players have passed into a parallel universe and inhabit each others bodies.  They now have to find the secret to swapping back otherwise they'll be stuck forever.  This works best if the players are randomly swapped multiple times, everyone should get a chance to play each other.  Pick 3 good and 3 bad traits (don't be too cruel though!!) about each player and sit back and let the fun begin.

Survival Game - If in doubt run a survival themed game.  My players always react well to a bit of zombie slayin'.

How do you do it, share your thoughts, tricks and ideas.