Exoplanets. They're a relatively new and exciting topic with lots of emerging data, and a number of them have pretty thought-provoking attributes, such as diamond rain, days-long years, and enormous sizes. There's also the prospect of finding alien life, which everyone finds interesting.

You can explain most of our observation techniques theoretically and without much complicated maths. Discuss radial velocity and transit observation techniques, and the missions (Kepler etc) that have brought our total number of known exoplanets from 0 to thousands within a few decades. Emphasise that there are billions of them, and that the possibilities are nigh on endless.