I'm likely seeing this too late to help with your event, but here's something I've found works well in a demo.
When I'm showing Stack Exchange to someone new for the first time, I prepare a question similar to one that already exists. So I start by talking a bit about the purpose of the site (focused Q&A, expert answers, etc.) and then go to ask my prepared question.
I type in my title, tab out of the title bar, and then explain how the system identifies questions that are similar to what I'm asking. The point being that the site may already have answers to the questions your potential new users have. This view is also a good time to explain accepted answers and a bit about voting, since the "Questions with Similar Titles" list shows some of that information about questions.
From there I sometimes talk a bit about community moderation and the privilege system, explain how a question can go on to help hundreds of people (and demo a question with high views to support this), and possibly give some statistics on how many questions have been answered. This may also be a good time to go into a more detailed explanation of the site's scope and show some example good questions.
If you're doing a one-on-one demo or if you're feeling brave otherwise, you can ask if the person/people you're demoing to have a question and then guide them through (optionally creating an account) and asking it.
Hope this helps.
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