Danube
The Danube flows through many cities, including four national capitals (shown below in bold), more than any other river in the world. Ordered from the source to the mouth they are:
The Danube is navigable by ocean ships from the Black Sea to Brăila in Romania and by river ships to Kelheim, Bavaria, Germany; smaller craft can navigate further upstream to Ulm, Württemberg, Germany. About 60 of its tributaries are also navigable.
Donausteig (Danube Trail)
The Donausteig is a hiking trail that passes through parts of Bavaria and Austria. Since the summer of 2010, the trail follows both banks of the Danube, from Passau via Linz up to St. Nikola and Grein.