I'd recommend something that they can play around with themselves outside of class. While you might get them to a point where they can understand a certain topic, it's much better if you can make them think about it beyond the scope of your presentation.
Most introductory finite combinatorics is good here - the binomial theorem, catalan numbers, derangements, graph theory, etc. There's plenty of room to give motivations, fun stories, history, etc. in this field to make the subject more human, which I think is super important when teaching to younger students (i.e. up to and including college freshmen/sophomores).
Probability is also a good supplemental topic with lots of counterintuitive questions you can ask them which they'd have a shot at puzzling out. (E.g. Monty Hall.)