With ZK and his family we have spent 3 days in Vienna. If you are interested click the map below to see journey GPX track and photos.
We visited Schönbrunn Palace (a former imperial summer residence), Stephansdom, and Kunsthistorisches museum (where one can see Pieter Bruegel's famous paintings including: The Tower of Babel, The Fight Between Carnival and Lent, and The Hunters in the Snow).
We also visited Flak Towers which are enormous concrete bunkers constructed during 2nd world war. FLAK stands for Flug Abwehr Kanone (or Flugzeugabwehrkanone) and Flak Towers were a part (not particularly effective--mobile system is always better for obvious reason) of German air defence system intended to protect cities and residential areas from Allied air raids. Their construction was planned in September 1942 and Vienna was the third city after Berlin and Hamburg that got them.
The Flak Towers fulfilled two purposes (cf. Flak-Türme Towers, Vienna: Nazi concrete heritage at Vienna's heart): They held cannons and spotlights that should fight airplanes from the ground; and they were important bunkers with an autonomous electricity, air and water supply system. They were built in pairs, comprising of larger Gefechtsturm (Combat Tower) and a smaller Leitturm (Fire-control tower). The three pairs of Vienna can be found in the Augarten in the second district, the Arenberg Park in the third district and one each in the Esterhazypark in the sixth and the Stiftskaserne in the seventh.
We visited Augarten and Arenberg towers.