I've done two career days, and they're always fun.

When I go in, I usually don't want to talk about the finer, more boring aspects of my job (doing beat checks, etc.) and instead just give a brief description of what I do and how I got into it.

After about two minutes of talking about myself, I just leave the floor open to questions for the rest of the class period. You will find no shortage of questions. I am barraged with the most interesting questions, from, "Why can't you say vagina in the newspaper?" to "Ever seen a dead body?"

I try to answer the questions as truthfully as possible, but also with as much humor I can pepper it with a more laid-back attitude. I've found that this is the most successful way in engaging a younger set that is more than likely disinterested with every single career that crosses their paths on those days.

By the end of the day, I got comments like, "Wow you have the coolest job!" or "You were the best one today!" I even signed some autographs that even those unfortunate kids didn't deserve. :)

At first glance I appeared the most boring speaker, especially since I lacked any PowerPoint presentation or live demonstration of what I do. But it was what I talked about that kept the kids interested. The teachers always try to get me back at the schools since they say the kids still talk about my job, and one of them I spoke to signed up for the local internship.

Mention how much journalists get paid -- they're bound to be impressed, unlike adults!

I am probably way late on this, but I did a lot of these at my past job. One of the best things you can do is bring that day's paper and point out what you contributed to it.
Another thing that kids tend to be "wowed" by is the daily miracle, and that someone is at the paper 24/7, pretty much. Because you have the circulation people and the ad sales reps and the reporters and the night copy editors and then the press workers. There is somebody there all the time. Kids seem fascinated by this.

It's never too late to comment :) And that's all great advice, thank you!