Heilbronn's astronomical clock sits on the old town hall facade facing the market square. It is a piece of 16th-century civic showmanship, with time, moon phases, and moving figures packed into a facade that survived in outline despite the city's wartime destruction.
It is a short stop, but a good one. See it with Kilian's Church across the square, then use the market area as the starting point for the rest of the centre.