Visit the Upper Belvedere Palace where the more modern art is housed (depending on your love of art, you can spend anywhere from one hour to a full day here).
Other must-see's on a limited schedule include the Hofburg Imperial Apartments and the Treasury (rather than the Silverkammer). The Treasury provides more in-depth information about the Hapsburgs and the history of Austria (great for history buffs).
Of course, visit the lovely Schonbrunn Park, which is open until dusk. There are other scenic parks that remain open until dusk including Volksgarten, Stadtspark and the Stephansdom, which is open until 7pm.
Other interesting stops include Karlskirsch ( St. Charles's Church) with its magnificent pillars and a visit to the Opera House . Even though the opera tour may be a bit crowded, it is certainly worth it. It may be a good idea to email the opera house to see if tours are being offered at times other than the usual 2pm and 3pm departures. (During the main tourist season, the opera house will have additional tour times.)
At night it’s fun to ride around on the trams, especially the 1, 2, and D, to see Vienna's beautiful buildings. Free Map with details of city center tram tour here.
** Note: The original information for this thread came from the following forum thread: http://www.tripadvisor.com/ShowTopic-...