Things that seem trivial or stale or just very familiar to you as someone who thinks about mathematics every day will be new and fresh and exciting to kids who have never seen them before. Examples include symmetry groups, Euler's formula and the Argand plane, the quaternions (and rotations, and the soup bowl trick), continued fractions, infinitesimals (don't underestimate the the thrill of "breaking the rules"), the logistic map and chaos... If you're not sure exactly what to do, don't put all your eggs in one basket by planning a rigidly structured programme. Just show them a bunch of cool stuff, and if something really catches their imagination you can take it further.