What is a good sample class to demonstrate TDD?
How about using the 1st section of Kent Beck's Money example. It starts out very simply but when you arrive at addition of two different currencies, TDD suddenly shows you the falisity of up front design, or YAGNI (you aren't going to need it).
Another good example is uncle Bob's bowling score TDD example. I think this is a good example of how a TDD narrative brings you to a clean solution that would have been explicitly unapproachable via an up front design.
To make it a really exciting presentation, up-front you could challenge the audience to design the two scenarios using whatever methods they find appropriate. You then would show the TDD way of designing them.
The real WTF moment for me with TDD was when Beck removed the two subclasses of Money, and the tests worked. This is not a trivial action; the man deleted two classes! The confidence to do something like this can be found only by two means.
1) gathering all the senior players in a code base and running through scenarios, followed by an extensive follow through to confirm it works
2) TDD
=D
If you have time for it, I would pick an example with an external dependency of some sort that is going to get abstracted in the test. Either a database, calls to a GUI, calls to a remote system, etc.
The reason is that one of the blocks to TDD is that the example seems too self contained. "Sure, when everything is a self-contained unit you can unit test, but when I have 15 systems to integrate, what is the point?" kind of thing.
I would also at least show one example at the end (look at Michael Feather's book Working Effectively with Legacy Code for how-tos) of migrating an existing class to bring it under TDD. Don't dwell on that as an example, but odds are your audience will be thinking about how to migrate the class they wrote that morning, no reason to let that fester as an "unmentionable."
TDD problems has a list of problems, ranging from simple to less simple.
Some have a list of tests to start from no solution yet.