My older sister, who auditioned for Kaplan, taught her five minute lesson on the conjugation of Czech verbs.
On the other hand, I taught mine on the construction of death-traps for spies invading your evil lair.
What we are looking for are the things that make a good teacher--you must be confident in your knowledge of the subject, you must be an engaging presenter, you must be able to elicit student participation, and you must be able to adhere to the five minute time limit.
Best of luck with your audition!