How about this:

I think I will bring a modded case and run the presentation from it. I'll explain all the pieces inside, and talk about what customization (inside) you need in order to do what you want - gaming, developing, plain old surfing, etc., and how easy it is to add ram, new video cards, sound cards, etc. Planning on making this about 20 minutes of the presentation.

Then, for the last 40 minutes, I'll show them a buncha cool sites they can create, with examples, and maybe some hands-on. I'll show them what the source code looks like, and make some changes on the fly so they can see how they can make "any" page look like "anything".

Then, I'll tell them if I'm invited back, I'll show them how to mod their own case. Since it's a nonprofit, maybe I can find a supplier who will donate some basic parts.