Here's stuff we HAVE tried that works well.
1) Pinkerton "How to Write Suspense" and Gabrielle, "Guide to Being a Groupie." I took the WikiHow assignment from last semester, co-opted it for causality (chains of logic, etc) and had them read these two creative how-to's, both in BANRR 2003. First of all, the students LOVED the reading, especially the Pinkerton. We talked about causes and effects, then moved into humor and cliche with the Pinkerton. I had them blog writing their own "how-to's," and it was an assignment which inspired some of the more hilarious and interesting writing of the semester. They get really into writing these things, and you can make important connections about logic and step-by-step.
2) Sammy Harkham, "Poor Sailor" works excellently for teaching lots of layers of things. I taught gutters and transitions, as well as causality.
3) Harry Frankfurt's On Bullshit works really well for teaching definition. Some of the things we cover in the discussion are - how does Frankfurt define bullshit and do you agree with this definition?, how does bullshit compare to lying for Frankfurt, how Frankfurt is setting up his argument, and whether Frankfurt is bullshitting us.
The New Yorker review of On Bullshit also works well for teaching evaluative arguments as well as argument analysis. Plus the kids get to say "bullshit" in class, which is sooo totally awesome.
4) LivingLikeWeasels Jamie
5) All The King's Men paragraph Have the students get into small groups. Each group gets a copy of the same long paragraph, and at the bottom of each page is a different audience prompt. The assignment is to rewrite the text to make it appropriate for their specific audience/purpose, keeping in mind things like tone, structure, and pertinent information. Some prompt examples: a women's magazine, a bedtime story for a small child, a blind item gossip column, a class lecture in the Forum building, etc. At the end of class they present their rewritten texts, often with entertaining results. This exercise could be done with any chunk of text; I chose the Robert Penn Warren paragraph because it's chock full of details.
6) Punctuation in practice/Comic timing: I hate giving punctuation lectures/exercises. But punctuation is still an issue that really needs frequent attention. Here is a link to one of the days when I picked just a few sentences from one of David Sedaris' essays, and we talked about changing the meaning through changing the punctuation. It's currently in mutilated form. While it did prompt some dubious semicolons, it really got them to think about punctuation as a CHOICE rather than a set of arbitrary rules to follow.