Ask a local what the most well-known structures in town are and they'll probably mention Danube Tower. It's one of the first things that'll catch your eye when you begin sightseeing.
Located in Donaustadt, this modern piece of architecture is worth including in your schedule. But you'll find tons more to explore in the area. Schönbrunn Palace, Vienna State Opera, and Hofburg Imperial Palace are within easy reach of Danube Tower.