That kinda depends on whether you're just interested in basic computer literacy type things, or more geeky stuff.
Assuming just a "find your way around" lesson, I would suggest going over the major bits of the interface that are different: How the Dock works and how to modify it, the menu paradigm (that is, system-level menubar rather than attached to each window), a basic tour of the Finder (how to make and adjust windows, the three views, get info), really general filesystem stuff (Applications folder, User Folders, and what the difference is), and probably a brief rundown on Safari and iLife if it's handy.
If you want to take it one small step farther into techie territory, explaining, the difference between User and System Libraries, how permissions work, and maybe a VERY brief introduction to the UNIX underbelly (more conceptual, maybe a peek at Terminal) might be good.
