The Danube Tower (Der Donauturm)
If you do not fancy walking up hundreds of steps in order to get amazing views of Vienna (like you will have to in order to reach St. Stephan's Cathedral viewing platform) - this sight is exactly the right one for you. The Donauturm is a real highlight in Vienna's "top attractions" - it opened in April 1964, and is the tallest free-standing structure (252 metres or 827 ft) in Austria, and among the 75 tallest towers in the world! The location is wonderful, right next to the Vienna Prater, and the views on Vienna, the Danube and the surrounding hills Leopoldsberg and Kahlenberg with its wine taverns and Viennese Woods are astonishing!
Two high-speed elevators take you to the viewing platform at 150 metres (490 ft) - this will only take you 35 seconds. You can also walk about 779 steps to the platform, but we would definitely not recommend it, this is not why you are here.....!
Many like spending time on the "top of the world" and really enjoy the thought of taking their time. The Donauturm offers two revolving restaurants that invite you to enjoy fantastic views over Vienna - be aware that it takes the platform either 26, 39 or 52 minutes to complete a full revolution. Both restaurants (one kind of snack restaurant, the other one has an extensive menu) offer good and basic food, nothing out of the ordinary, but definitely worth it in case you decided to take a break.
And there is of course something for the extreme visitors. Some years ago the Donauturm decided to offer one of its decks to use it as a bungee-jumping site, with a platform projecting from the area. You might be lucky and able to observe some of the daring ones, but the platform is only opened and used at times during the summer months. The weather needs to be right too....check out the website if you are interested!