I propose: A song.
Written a verse at a time. Topic and context will drift, but the kicker is, it must be logically connected to the previous verse somehow.
Each poster completes the previous verse, and the first line of the next.
(So, if each verse is ~ four lines, I'd write line one, then the next guy would write lines two to five).
I my mind the verses will have the "Irish Drinking Song" rhyme scheme, or ABCB, but don't feel restricted by this!
I'll start with one line! (Don't mind multiposts, but wait a decent amount before recontributing!)
"I once saw a diver!"/