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.
Sziget Festival
Being located on an island, some festival goers have tried to enter by swimming across the Danube or by paddling across in inflatable rafts. The organisers very much discourage these attempts as it is dangerous due to the tricky nature of the fast-flowing Danube river.