If you want to get really meta, you could teach how to prepare for a presentation.

Other unedited brainstormy thoughts:
Booking a trip
Preparing/planting a garden
Childbirth (maybe with an animal instead of a human?)
Developing film
Preparing for an exam
Mail merge, PivotTables
Making beer/wine/spirits/cheese
Communication
Card games are a process - maybe a less-common one like canasta

I see I am also veering away from professional topics. I think this may be because a lot of professional processes are pretty complex and hard to do justice to in 10-15 minutes. More professional processes:
Project management
Software development (SCRUM? Is that a thing?)
Instructional design
Process improvement (PDCA, Six Sigma/DMAIC)
Performance reviews
Hiring
Coaching

I had to give a teaching presentation to get the job I have now. I gave a talk on managing up (basically, managing your reputation with your boss and others).

There have been a few threads on MeFi (1, 2) about this, people interviewing for the Princeton Review. Usually the best options are ones where there is some sort of built in interaction with the audience, a fairly straightforward how-to lesson and something that actually teaches people something they don't know. So instead of "how to bake cupcakes" which many people have probably done, yo ucould do "How to bake cupcakes for 500" or something which will involve something they know and something they don't know.

When I did a talk like this in a Princeton Review vein, I did "How to season a cast iron pan" though I forget what the audience interaction part was.

Yeah, I saw the Mefi threads, but this presentation isn't for Kaplan or PR and those seemed designed to be more fun, per se. I don't need an audience interaction element. How to season a cast iron pan is a good one though--thanks, Jessamyn!

Jeoc, those suggestions are great--thanks!

Oh, I should say, some other topics I considered are how to set up a wiki and how to use RSS feeds for research.

How to bake bagels.
How to test if a large integer is prime.
How to pick a lock.

When I had to do this on the fly without prep, I took my interviewers into the bathroom with me and taught them how to wash their hands correctly. I got the job.

If you are looking for something with audience interaction that is easy to do well, teach them how to make paper planes. I once did this and I drew all the fold lines on a piece of paper, then copied it onto lots of different coloured paper. The end result was a room full of brightly coloured planes and lots of smiles. If nothing else, they will remember your presentation fondly and, if you do it well, they'll also remember that.

Dispose of a body! (Well, it gets you away from cupcakes).

I think I'm going to do why/how to set up a wiki, if I can think of a way to compress that into ten minutes. Thanks for the suggestions, guys.

How to fix a flat tire on a bike. You could actually bring a bike in!