The old town is small and best walked: the cathedral, Place du Vieux-Marché, Saint-Maclou, and the Beaux-Arts museum are all within a fifteen-minute square. A modern tram-train (the Métrobus) connects the station to the south bank of the Seine, useful if you are staying on the left bank.
City buses are reliable but rarely necessary for sightseeing. For day trips into the Norman countryside, a rental car is by far the easiest option; the station has several agencies.