So I thought I'd share a few ideas (of the sort that I used to do a while ago, in China), for all those on Dave's who've ever in the past asked for potential Halloween activities (sorry that my indirect reply is coming a bit late!), or who might ask in the future.Something that doesn't require too much preparation is telling ghost stories such as 'The Monkey's Paw'. If students aren't familar with "making wishes", you could lead into things by mentioning Aladdin or some other context that involves this function and its prototypical form. I'd then put on creepy music (tied to and timed to coincide with the storytelling), for example, start with A Nightmare on Elm Street Part 2: Freddy's Revenge's "Kill for me" track ( http://www.youtube.com/watch?v=dhxEP-tC4b0 ), proceed onto the opening stuff from Pet Semetary for when the story becomes sad, and end with something to evoke a building storm and wind (maybe a bit from The Serpent and the Rainbow? Can't quite remember what I used here) for the story's climax. Anyway, here is the rough version that I tell (I often simplify or cut vocab and/or convey meaning by noises, gesture etc):
Once, there was a man, his wife, and their son.
One day, a friend came for dinner.
The friend had been to India.
He gave them some presents: tea, spices, clothes, silk...and a dry old monkey's paw.
'(They say that) this paw will grant you three wishes' he said. 'But I've never tried it myself. (Pah! etc)'.
After the friend had gone, the man asked his wife and son what they should wish for.
'How about some money?' the son suggested.
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So the man took the paw and said, 'I wish I had a thousand pounds!'.*
They waited and waited, but nothing happened. No money appeared. So they went to bed.
The next day, the son went to work at the factory, the father sat reading his paper, and the mother started preparing lunch.
Suddenly there was a knock at the door.
It was somebody from the factory with some awful news: the(ir) son had died in an accident. They were/the factory was very sorry, and hoped that this thousand pounds (from the insurance) would help in some small way.
The son was buried, and they were very sad.
Then, back home after the funeral and as night fell, the wife suddenly remembered the paw! It had killed their son, but maybe it could also bring him back! So she found the paw, and made their second wish:
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'I wish my son would come back to me'.
Again, nothing happened, but it did begin to rain, and soon there were flashes of lightning and rumbles of thunder - a storm was coming.
But wait - what was that? The wife thought she could hear a knocking at the door. Yes, there it was again! A heavy, slow knocking! She lit a candle and ran down the stairs towards the door, crying, 'My son! I'm coming! My son!'
The man, woken by all the noise, saw the paw, and immediately knew what his wife had done. Then, hearing the ever-louder knocking, he could see the son in his mind's eye/imagine the son, not as he was before, but as a twisted, zombie-like ghoul. So he picked up the paw and made their final wish before the wife could open the door:
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'I wish my son were dead again'.
The wife excitedly drew back the final bolt and threw the door wide open. A cold wind howled in, and outside, all there was was the dark, empty street.
The dotted lines above indicate points at which you might like to pause and ask the students what they think the wording of each wish would/should be like. Regarding the asterisk, a thousand pounds was a lot of money "in them days" (the mid to late nineteenth century), right? I sometimes even have the son or father say something like 'But not too much (=let's make it a realistic amount, so it's more likely to come true)' before the first wish is made...but make it millions in your version of the story if you like (assuming your average insurance pays that much even nowadays!).
Don't be afraid to retell stories, give students gap-fills once (or indeed before) they're familar with the spoken version, or ask them to write and/or tell their own version etc etc.
My "Vampire Taxi Driver" comic strip story is also pretty easy to prepare. It's nothing that amazing, just a bit of lighthearted fun that I knocked up quickly once for some children/pre-teens. They came up with some good continuations of the story e.g. one group drew pics of the guy returning home, going to bed, the vampire climbing in through the window but luckily being killed Nosferatu-style by the rising sun just before he could sink his fangs into his sleeping victim, and finally the guy waking up to find a smouldering vampire corpse on the floor next to the bed. Pretty self-explanatory, in other "words"! The link is here (not sure it will print that clearly though, but any would-be artist should have little trouble in redrawing the pics themselves):
(EDIT - NB: Please be aware that Imageshack may be harbouring PC scam virus-scan pop-ups and the like nowadays)
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Next, pics like those in the Dark Cults card game (I really recommend you hunt the complete game down, perhaps on eBay or wherever. Great game!) can provide an "attractive" and supportive way of getting the students' imaginative juices flowing (pictures can be easily understood, so the receptive burden at least is eased considerably, unlike with my "13 Scary Slips" activity below). I tried using just 15 cards (out of a picture deck around four times that size in total) with some friends, and these gave us enough characters, settings, events and generally suggestive pictures to enable us to make a story that it was decided had to involve me (fluffyhamster) booking into a hotel, overhearing creepy conversations, being unable to sleep, spotting then following a procession of dark figures out into the woods, seeing digging, suddenly finding some of the cowled cultists behind me, being grabbed, asking what they wanted, before finally being told simply 'YOU!' (presumably for a sacrifice and/or being buried or whatever. Maybe they were even digging a monster out of its slumber, in order to feed it!). Kinda fun, eh!
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Or you could try something like Morgan & Rinvolucri's Fortunately/Unfortunately story-building game, in which a central character is decided upon (often a teacher) and subjected to alternately good and bad events (which makes it a bit like Dark Cults!). The starting situation or event can be either good or bad or just plain mundane - if mundane then the next turn can and must be an actual development, either good or bad - and play then alternates between good and bad developments, depending on what type the previous one was. Play can be set to end within a predetermined number of turns, or at any time after a bare minimum. An example I witnessed: Teacher X was sitting in his garden when a UFO landed > Some aliens took him prisoner! > One of the aliens had the face of a beautiful woman > But unfortunately the body of a man > But this was OK because Teacher X is gay! The students don't have to write their suggestions, the teacher can just board them in a rough descending sort of side-to-side zig-zag. If you want to see what M & R themselves suggested, details can be found in their Vocabulary activity/teacher resource book (OUP) IIRC.
Finally, my "13 Scary Slips". Requires reading and writing, and possibly some cutting up. Say that you have a story for the students before quickly teaching or checking at least the following vocab (don't do too much more though, unless you know the students definitely won't know or recognize the other words in the story, because it might spoil the "mystery" of the activity; you're just whetting their appetite at the moment): lightning, thunder, storm, divorce, disappear, killer. The students will probably be able to start inferring what kind of story it is. Then give them the following story in order and complete on a single sheet of paper (if you'd prefer that they try to get more quickly into continuing the story); or on a single sheet to each student but with the sentences except the first all jumbled (to fill a bit of time); or hand each group the story in the form of thirteen jumbled slips and ask them to re-order it (it helps if you ensure and tell them that the top slip is in fact the beginning slip) if you'd prefer them to grapple with issues of texual cohesion (or if you really do need to fill quite a lot more time LOL). NB: the asterisked sentence could appear in a few other positions.
Actually, some of the other sentences too perhaps - you might decide things need rewriting/tightening up a bit. And obviously some of the vocab could be omitted without loss to the story, though that would reduce the input. 
A man and his daughter had been to a party.
The party had been at a friend's clifftop home, miles from anywhere.
They got ready to leave. They hadn't talked much to each other at the party, which was unfortunate, because they hadn't met since the divorce when the daughter was very young.
They got into his car and drove off.
It was very late, and very dark; there was no moon that night. It would be a long drive home along the old coast road.
After he'd been driving for a while, he asked her how she and her mother had been getting on since the divorce many years before.
* Lightning flashed in the distance. Seconds later, thunder rolled over the car as rain began to hit the windshield.
She told him that actually, her mother had mysteriously disappeared two years earlier. She was sorry that nobody had told him about it, but neither of them had liked the mother's strange behaviour anyway.
The car suddenly seemed much smaller. He sighed and turned on the radio.
A few minutes later, there was a newsflash on the radio. A dangerous killer had escaped from a mental hospital. Police were looking for...
The daughter immediately turned it off - on a night like this, she didn't want to hear anything frightening!
Just then, the car went out of control for some reason and crashed into a ditch.
The man was badly hurt.
WHAT HAPPENED NEXT?
Here's an example of what some of my students ("students"? I didn't have to correct or improve things that much!) came up with:
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Another group had an ambulance arrive to rescue the father and daughter, but its driver turn out to be the mother (she rather than the daughter was the escaped killer!), who then drove the soon trussed-up and screaming pair off into the night.....
