Most important topics for a short introduction to Prolog
My idea is to teach them enough to write simple provers for (fragments of) the systems of logic used elsewhere in the course.  I had lots of experience writing interpreters from other courses in my curriculum, so I would have been prepared for that sort of approach.  Introduce SLDNF as the basis of sketching the limitations of using prolog in such a fashion, so they can reason about what's going to be hard and what's going to be easy to do in prolog for the remainder of the course.  Of course, my approach needs prolog to be introduced early instead of the last week.  If you like this approach, I suggest giving them libraries to render the proof trees or whatever that you use in the rest of the course.