The beauty of being in a city full of architecture means you can spends your days sightseeing for free! Vienna has plenty of buildings to admire including favourites such as Stephansdom, the Hofburg, Schloss Belvedere, Das Rathaus, Schonbrunn Palace, Hundertwasserhaus and the list goes on! You could easily spend a few days moseying around the city, using your unlimited trip rail pass and munching on a Bratwurst. All of this will probably cost you just a few Euros!
For some more detailed travel information on Vienna, please see my other post here.