Vienna keeps its skyline in one place, on the far bank of the Danube, and this walk is the cleanest way to see it: a river island on one side, a wall of glass towers on the other, the water between them. Three and a half kilometres, an afternoon, mostly flat and open.
The U1 lets you off mid-river on the Reichsbrücke, with stairs straight down onto the Donauinsel — the 21-kilometre island built to stop floods and adopted by the city as its beach. Walk the waterside path; in summer it's swimmers and barbecue smoke, and the Copa Beach strip on the east bank turns loud and cheerful. You can stop anywhere; nothing here is fenced off.
Cross back over the Reichsbrücke toward Kaisermühlen, where the city's only real skyline waits. The DC Tower, Austria's tallest, heads a cluster of glass towers with no trace of old Vienna in them — which is the point of coming. North along the water is the curved Vienna International Centre, the 1979 UN complex everyone calls UNO-City. End beside it at Kaisermühlen-VIC, one stop back into town under the river.
