A huge swimming and recreational island in the middle of the Alte Donau. It is a Viennese institution, complete with chalet-style cabins, together with well-tended little gardens (the waiting list for one of these is seven to eight years, so tourists need not bother), its own fenced-off naturist sector, numerous bars. There are also swimming facilities in a variety of swimming pools and a lake (which gets a bit grubby later on in the season) and plenty of grassy areas for lying around and chatting with friends. Open from the beginning of May till the end of September.
Easily accessible by Underground U1 to Vienna International Centre, then 5 mins by foot
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The advantage of the Gänsehäfel is that when the rest of Vienna is stifling and the temperatures are pushing the high 30s, the enormous old lime trees and the water of the old Danube mean that it's five to 10 degrees cooler here than elsewhere. Perfect place to flop out when it's too hot to think.
Don't miss the great ice-cream parlour on the corner of Schüttauplatz on the way home.