"Chain Stories"
Post a topic called Chain Story (7 Words), or however many words you want to use. Whoever starts obvious types in seven words and then the next person posts seven and so forth. Every 16th post starts a new thread page, so if your the 16th poster you need to copy and past all the story so far and then proceed it with your seven (or however many) words.
Ex.
Bob: There once was a man who had...
Jeff: Three gnarled toes on his left foot...
Susie: But when the fish saw him swimming...
Milford: They all went to nibble on his...
Frank: Pumpernickel sandwich with pickles and green olives...
Etc.
[Yes this is a horrible example, hence why I haven't already started one.]
Another of Course is:
"Word Association"
Someone posts a word, the next person posts the first word that pops in their head. We pick a number to play to and when we reach that many posts we see what word we started and ended up with.
Milk -> Ceral -> Spoon -> Fork -> Knife -> Stab-> Bleed -> Hemorrhage -> Anaphylactic Shock -> Static Cling -> Etc.