I once learned how to juggle 3 things pretty well over the course of a 2 hour TV show
I also learned how to juggle in a week or so. By 3 weeks, my friend and I could pass the bags between each other fairly consistently.
One more tip: use beanbags not balls to learn. Beanbags don’t roll away when you drop them.
Here’s a really simple one.
Get a couple of pieces of dry pasta (spirals work best) - just before you’re up front, slip one into your mouth and hold it between your back teeth. Put your hands on your head/neck and pretend to be twisting it, whilst slowly crunching the dry pasta (with your lips closed) - the audience should be fooled into thinking they’re hearing the bones in your neck cracking and grinding.
I came here to suggest juggling, but several people beat me to it. I learned the basic 3-ball cascade over the course of maybe a week using this book.
(Let me preface this by saying I’ve worked in China a few times before).
Although there’ll be some people impressed that you tried to recite a Chinese poem, there may be some jerks who snicker at your pronunciation. If you feel you have to do that then make it a very well-known piece as Chinese is a very tricky language to learn to speak, especially to Chinese people who aren’t accustomed to a westerner mispronouncing/murdering their language.
The mime thing might be good - Mr Bean has a huge following in SE Asia - but then you would have quite the act to follow. I hope the setting fire to your hand idea was a joke :eek:
Do you practise any sports? Being adept with a basketball would be a thought, depending on your age, as its a very popular sport in China. Actually basketballs are big and heavy; you might break a light fitting or someone’s nose!
You could learn a monologue in English. Something famous and classical (“Alas, poor Yorick” etc); the few who pick it up will be impressed you know it and feel proud they recognized it, the others will be impressed/confused but won’t think less of you. If you fluff a few words no-one will be the wiser 
Learning to juggle might be suitable for your hand-eye coordination, and lots of fun and therapeutic to boot by the looks of it. Telling them you started learning only 3 weeks ago will mean you save face and everybody will be more relaxed.
If you think their English will be good enough to understand simple commands you could teach them some very basic moves in Tai-Chi or Yoga, or teach them to say “Hello, how are you?” in several other languages. Unless they’re very familiar with Italian, French, Spanish or German (European languages will go down best) they won’t be able to fault your pronunciation. You can have them all repeat some phrases back to you, in unison; this is the way they learnt in school and so are most comfortable with and nobody loses face.
Finally, you’re not expected to teach them anything - it’s just an exercise in getting-to-know-you. Anything you do with some conviction and a sense of humor should be sufficient.
Another vote for juggling. Go to a toy store and get the Klutz book that comes with three bean bags. (They still make those, right? I haven’t seen a Klutz book in years.) You’ll be juggling by the time the weekend is up.
You could make jewerly, it’s very easy.
But regardless of what you do, will you come back and tell us? I am curious!
What I was thinking to do was this: find a poem where I can not only find accurate pinyin but also a video online of a chinese person reciting it dramatically.
Practice it a lot, and try it out on some friends prior to the big day (and I have friends critical enough to tell me “no, you’re still not saying it right” on the nth iteration, for any n).
And I’d slightly jazz it up by putting on a wig, say, to be confucius or whatever.
I’m not entirely sure about it, but it is the front-runner currently.
You betcha. But you might regret that…if I (figuratively) die on stage, I will share the pain…
Send and receive morse code.
I think there’s promise here. What can you play badly?
I’ve just had another idea: a seance! That’ll learn them.
Piano. I’ve never really had lessons, and the toughest song I learned to play was Loss of Me from FFIX. Even then, my timing needs work.
I could get that up to performance level, or learn something of similar difficulty, but I am sure there will be at least 3 people among the group that could kick me off and play something more impressive. That’s the problem with the piano.
While we’re listing skills, I can salsa dance a little. I could probably work out a nice routine with 3 weeks to practice, but there’s no immediately obvious partner.
Hmm…it would be a great excuse to approach some of the women in the office though… 
Let’s be clear here. Do you want a goofy lame trick to amuse your co-workers, or a really skilled one to impress them?
Balloon modelling- so long as you get the proper balloons (and a pump), the dog, swan and flower are all really easy and recognisable. Give everyone one to take home!
Use an orange to power an LED. You will need copper wire and galvanized nails.
I have taught hundreds of speech classes over the years, and this sounds a lot like a typical “Demonstration Speech” - where your purpose is to not only show what you can do, but show others how to do it.
I have seen a lot of fun demonstration speeches that might work:
Prepare a food they wouldn’t know - might be something as easy as making an “American” tuna fish salad, and let the more adventurous taste it. Or guacamole, or if you can bring a hot plate and skillet - maybe something like French Toast or some other specialty you remember as a kid but they might never have heard of.
In the same vein, perhaps some kind of fruit drink/smoothie with a blender? A milk shake? I am not sure what would be considered “exotic” drinks there, but I am sure there is something you can think of that would make them go, “wow” and want to try it at home.
Doing balloon art is not all that difficult to learn, and you can pass out balloons and let them do it with you. You should be able to learn a few quick things just using YouTube.
How to tie-dye a tee-shirt - simple to do, have a few finished examples and take them through the steps to do it at home alone.
There are several places online you can learn how to create hiding places for valuables in your home - how to turn objects hollow to hide money and jewelry, or where to hide such items in a freezer or other odd locations. Then again, you don’t want the police showing up at your door the next day, so I guess that depends upon your audience and local laws…
There are tons of simple, science experiments online as well - often used in school rooms but still fun to show others in a group.
I had a new mother show how to child proof a house.
There was a guy who worked at a sports studio who showed exercises you can do in your office, at your desk - very fun and many were joining in.
Simple, but new to many, was how to set a table for a fancy dinner - with the forks and spoons and knives - explaining what they were and why they were placed the way they were for each course. This might be new to your audience. That person also showed the best way to open a bottle of wine.
One guy showed how to draw Mickey Mouse - very easy and fun and the results were amazing!
Another student showed how to cut fruit and vegetables that looked like flowers and cool designs for salads and decoration on tables.
So - not sure if the demonstration speech would be appropriate, but you would only have to Google “demonstration speeches” to find whole bunches of ideas you could use.
Update
Well, I promised an update, but I’m sorry to tell you: I didn’t do any talent. Sorry. 
<excuses>
First of all, they cut the time down for each person to 3 minutes. Just reading my introduction in Chinese would fill that.
Also, I got screwed over by the Chinese public holiday system.
Basically, when a public holiday is coming up, it’s common for Saturday and/or Sunday to be working days so that they can smoosh non-working days together.
It’s difficult to explain concisely, but with OT I’ve ended up working 11 days consecutively and had sod all time to prepare anything anyway.</excuses>
In the days leading up to it I tried to practice breakdancing, but at the age of 34 it didn’t come back to me as easy as I’d hoped.</another excuse>
So I just did the speech. Which went down fine.
Sorry for lame ending to thread…
Well at least tell us you still wore the wig!
There are a whole host of TED talks that are under 5 minutes that demonstrate how to do a common procedure, but introduce a twist to it that vastly improves it. Did you know that a huge percentage ofpeople are tying their shoes incorrectly? Or are unable to dry their hands without using a bazillion paper towels? Just entering “how to” into the search box on TED’s website brings up some interesting subjects.
Edit: dang - only a month late!
Great links though, thanks!
Wearing a tux walk onto the stage carrying a folding chair and a CD player. Unfold the chair and plug in the CD player. Play a classical piece on the CD player preferably an aria from an opera. Sit and listen. When it is over, fold up the chair, unplug the CD player and walk off the stage.