This is a surrealist game I sometimes play when friends are over. It's going to be a little different on an online forum, but I think it might still work well. I'll set out the rules:
1. We will be collectively writing "poems" consisting of 15 lines. We will take turns adding lines. There will be no particular order, but each contributor, after adding his first line to a given poem, must allow at least three other people to add further lines before adding another line to the same poem.
2. Each line (after the first one) must be an "opposite" of the immediately preceding line. The notion of "opposite" is to be interpreted very loosely. Do not, as far as possible, simply provide the obvious or direct opposite to a given line- for example, don't write "white" where someone says "black," or "up" when someone says "down." A more fitting opposite of "down" might be "sky," for example. This prevents the poems from being repetitive.
The silver cat retreats into the manger
While solar mice assaulted the ivory castle.
3. Try to continue each other's sentences- don't make each line a stand-alone sentence. Try to maintain the flow of a cohesive written piece even as the ideas get increasingly strange. Transition words, commas, and enjambments are your friends.
4. Number each line so everyone knows how many lines have been entered so far. The person who writes the fifteenth line should redact all the previous lines into his post so everyone can read the complete poem.