Science/​rationality subjects to teach

I recently got a great new job. In short, I organize short (3-9hour) courses on pretty much any subject I want and people can pay to take them. If not enough people reserve a spot, the course is cancelled. If this happens to much I get in trouble with my boss.

This setup gives me a golden opportunity to raise the sanity waterline from the bottom up as most of our customers are ordinary people of the street. I have a few projects I am working on already but would like your input: What are some easy to learn subjects that will get people interested in science/​rationality or at least more sane?

Some things to keep in mind: First, it has to grab their interest or they simply won’t come, something that they encounter in their daily lives, something the worry about, etc.
Second, it has to be concrete. Purely theoretical subjects have no success. They come in with clear questions and issues and want them answered, not because of some inherent curiosity about science/​rationality. They need to walk out the door feeling they have learned something valuable an useful to them.

So, thoughts?

Edit: My first courses are planned for October, I will report on their succes here on LW if the subject was relevant.