I suggest, when doing the mod example, to do a "this is how you do it, but in the interest of covering more material, I will skip and use something prepared ahead of time for this demonstration."
Then you can lead up to say, a mission, load it up, demonstrate how to make changes in the SEXP tree, etc.
Oh, and I'm not sure if you've done large presentations from a computer before, but if you havn't, remember:
Set up a custom user with large fonts enabled, all acessability options for readability on. Get used to working with WinXPs Magnifier(or download a 3rd party tool) to quickly show text. Try and check how colors come out from the projector you will be using. Don't assume your programs will work, make sure to have a backup, etc.