I've found that gratuitously showing off Exposé works wonders… demonstrating iChatAV might work pretty well too. People are generally very impressed with things like instant wake from sleep (even though technically you can actually convince Windows to do this, apparently).
Beyond that… well, it depends what she's interested in, I 'spose. It's difficult to think of "must-show" third party apps because most of what a normal user would want (apart from an Office suite) is already there.
Oh: and demonstrating PithHelmet might work well too.