The old market square is the historical and contemporary centre of social life in Rouen — the place where Joan of Arc was burned, the place where the modern church now commemorates her, and the place where most visitors find themselves at some point during the day. The covered market runs along one side; the half-timbered houses around the rest of the perimeter are crowded with cafés and brasseries.
The contrast between the medieval streets feeding into the square and the swooping modern church is the city in microcosm. Worth a slow drink at a terrace café even if you've already done all the sights.