So... Who'd be interested in this, and what would you like to see?
Yes, make it very basic and keep to some central points. My beginner's lecture last year turned out to be too crammed even though I tried to stay on the surface of things - 3D is just that broad a field. I would recommend making lots of examples and follow-through workflow. Like:
1. What are the basic concepts? Stick to modeling, simple texturing, getting a render out of the system.
2. Demo: Where do I find what in Blender? How do I handle the tree, do I work with drag & drop a lot? Which menus are most basic?
3. One simple model: From box to head (or something). Have some pre-made inbetween stages ready to show.
4. Executing texturing with plain color and simple patterns.
5. Get it lighted and rendered, only basic options!
If there is time, get some animation basics in it.
Do not (if going with a beginner's lesson):
1. Start with sculpting, fluids, specialties of Cycles (that would be a full lecture each)
2. Any complex animation. You can mention joints and IK, but I believe this would be too much stuff for beginners.
3. Anything that will require a deeper understanding as precondition. No matrices or gimbal lock issues
We probably need a yearly introduction to 3D in order to generate more interest in the topic.
Alternatively, you could do some specific lecture. I think about doing facial expression and animation myself this year, perhaps including speech and lip syncronization, hopefully on a level that can generate some interest even from non-3D people.
Specifics, though, always have these two issues: Not yet enough people in the fandom with 3D basic knowledge and skills, and too narrowly focused to attract interest from people who are not already familiar with the foundation. The best specific lecture IMHO would be balanced between the details of the field (knowledge) and the flashy show effects that result (entertainment).
So many topics possible...