The campus visit is also an opportunity to learn about the school. You can get a sense of the campus culture, and of the department. Don't think like someone who is desperate to get a job, any job, but rather like someone who might be sold on this particular job. Hopefully, the people you're interacting with are of a mind to sell you. Ask questions about the students, about campus life, about what it's like to live there. You will need to exhibit at least minimal chit-chat skills, because you'll be doing lots of it while people are walking you around campus, driving you to the airport or restaurants, etc. A five minute walk becomes much longer when filled with awkward silence.
Anon asks specifically about the job talk:
Is there any circumstance in which it's appropriate to give your job talk on the paper you gave as a writing sample? I work in a somewhat technical and obscure area, and am interviewing for an open job. And I sent my most accessible paper as a writing sample. And while I'm pretty good at explaining my technical stuff to a non-tech audience, I know the writing sample talk goes down a storm with varied audiences. But I'm a few years post-doc, and I don't want them to think I only have one paper!
You want your job talk to be something you know inside and out, and something you're comfortable presenting. If that's your writing sample, or a variation thereof, I'd say use it. (For one thing, it's likely only the SC read your writing sample, so for most of the audience, it's going to be new.) You certainly don't want to try out new material. (Although as a grad student, I saw a job candidate deflect many, many questions during a job talk by saying he needed to think about the issue, and come back to it. He didn't, of course, come back to those many questions during the talk. He also got the job, although he wasn't the first choice.) If you plan to use powerpoint or some such, ask in advance what tech will be available, and be prepared to not use it, if something fails. You have far more control, in some cases, over the content of your job talk than over the content of a teaching demo. I had one campus visit where I was assigned to teach half of a particular existing class. It was a course that was completely outside my AOS/AOC, and outside the AOS/AOC of the job ad. The SC chose it, so far as I can tell, because it was convenient -- the prof was willing to give up half of his class for the demo. But it was hellish for me, to come in during the second half of a class, try to bring the topic around to something I was able to teach well, and have only about 30 minutes to do it. (I didn't get that job.)
Take granola bars and portable snacks, especially if you have dietary restrictions. The days are really long, and being hungry makes you cranky. Have at least two pairs of good pants, especially if you're going to a wintry clime where the odds of getting mud/snow/salt on your pants are high. It's a short trip, so take a carry-on so nothing gets lost in transit. Be polite. Practice a firm but not crushing handshake -- you'll be shaking lots of hands. Be very nice to the department secretary/admin assistant. They know where the bodies are kept.
Jump in with fly-out questions, advice, or whatever.