Create a flow gadget or RSS feed or other auto-updating thing to give people a short and straightforward creative exercise to do, up to twice a day. In the style of OneWord as an iGoogle gadget, for example.
Could make it social with games like these:
* Write oneword type paragraph, then provide a (related) word for the next person. Show your previous posts in the string.
* Web 2 whispers - you write for an unknown random number of seconds, then the text is snatched away for the next person to continue - they get as many seconds as you had to read it, then a random number to write.
But really, variety would be a great thing. Get people doing word association, describing a day as an X, relating two words, perhaps even writing for the length of a particular song, taking a phone photo of some interesting bit of ceiling, writing a haiku, chewing an interesting shape, drawing on a banana, following on from a randomly chosen sentence, following on from other people's following on sentences, taking a photo of two and a half things, explaining their choice of colour for 'think', imagining the world without an X, thinking up new ideas about a randomly chosen product, venting their spleen, getting angry, sending an extract from a receipt in their pocket, drawing a both-ways-up face, designing a tshirt, spilling some water (intentionally) and giving it a caption, "You are a dog, what do you think of fridges?", asking any of a thousand useful (and perhaps user contributed) questions - what would make a train journey better?, things to do with paperclips/cds/receipts/short pencils/unwanted cushions, invent a new magazine.
How would it work? Just provide a stimulus for the first few people, and occasionally drop in the question: what would you ask of people?
Setup: Blog-style post-and-reply would work well, but the replies must become the centre stage, rather than the post, and it must allow for more complicated supply and reply mechanisms for many of the above games.
Ah! How about a dynamically generated RSS feed unique to each user, supplying a reply-type link for the responses, which updates to show the state of the game when they're finished.
Further rumination, generalising yet further: make the whole thing a game. Let people cluster towards fellow users who supply their kind of game or stimulus, allow them to form their own mini communities. I don't want to say 'Facebook app', but...