I think there is nothing that is both as elementary, useful and fun as elementary probability. Probabilistic thinking is relevant to decision making and extremely underdeveloped. Even Paul Erdös got the Monty Hall problem wrong, when he was first confronted with it. So probability is certainly not trivial. The amount of formalism needed is very small, so students afraid of complicated expressions will not be scared off. One can cover a wide range of conceptual and practical problems, from brain teasers, probabilistic paradoxes to how one should interpret medical tests.
I think there are some rather simple concepts not widely understood that should be really hammered into peoples heads such as (elementary) conditional probability, differences between causation and correlation, selection bias etc.