Motivating Talk on Foundations of Cryptography
Since you can't rely on a knowledge of complexity theory, you have to emphasize the change in paradigm from security by obscurity to security by intractability, by positing the idea that some problems are hard to solve efficiently. This of course elides the many problems associated with the Impagliazzo worlds of intractability, but it gives a flavor of the way modern crypto works. 

for ZKP, which are truly awesome, there are many ways to convey the basic ideas intuitively. See for example my answer on MO, as well as the hilarious Ali Baba and the 40 thieves story. While these were originally designed for a younger crowd, they work well at all ages to convey the right intuition.