The most clever trick he talked about was how Kepler estimated the orbits of the Earth and Mars (assuming them to be non-circular) using the multi-year data on Mars' declination collected by Tycho Brahe. This led Kepler to formulate his laws of planetary motion which led Newton to formulate his theory of gravitation. A great story of giants standing on the shoulders of giants...
Here's the slides in case someone's interested.
It was surprising to see how comfortable Tao was discussing a subject thats not his main area of expertise. He even fielded a few questions from the audience in the end.
All in all, a great treat!