If you take the U4 line to Heiligenstadt ("Holy Place"), you can jump onto bus 38A and head up into the hills north of Vienna.
Looking down at Vienna from Leopoldberg. The river is of course the Danube, with the Old and New Danube rivers on each side of the Donauinsel ("Danube Island")[c]. The Leopoldberg was the site of the Battle of Vienna[d], where the Holy League[e] routed the forces of the Ottoman Empire[f] in 1683.
On top of the Leopoldberg is this memorial to Austrian prisoners of war from the second world war.
View down toward Vienna from the memorial.