Depending on how many hours you have, what you are interested in. I would highlight the main points: Belvedere, Schönbrunn, Hofburg. In each of these places you can choose what you are interested in, an art gallery or a zoo or a treasury. In addition, walks through the old town, pedestrian streets, visits to the Cathedral of St. Stephen, there you can climb the tower, the Cathedral of St. Karla, Town Hall, ... Food and drinks are not cheap there, this year mulled wine at Christmas markets 4 euros, strudel 4 euros