The Donauinsel is a narrow artificial island, 21 kilometres long, built in the 1970s to protect Vienna from floods on the Danube. It is now the city's main outdoor recreation space: cycling and running paths the full length, beaches and swimming areas, bars in summer, and an enormous music festival (Donauinselfest) every June that draws around two million people.
It is also a great place to cool off in summer; the river is clean enough to swim in. U-Bahn line U1 stops at Donauinsel in the middle of the island; rent a bike for the long ride to either end.