A lot of culture, a lot of sightseeing. The castles and nature around the city are beautiful and this imperial city is a must see. I'd recommend visiting Belvedere, Kunsthistorishes Museum, Schoenbrunn palace, as well as Albertina (those are the most important sights). Enjoy parks and the Danube river! Grab some sun close to the Donau Insel subway stop, it's amazing! Partying is limited to Fridays and Saturdays almost exclusively and it's not too lively compared to other European cities.
Insider Tip:DO NOT, DO NOT and DO NOT pay 18.5 euros to go inside Schoenbrunn. It's too expensive for no apparent reason! Plus they are disorganized and distastefully expensive!!! Just enjoy the park around it (for free) and try the labyrinth for just 4.5 euros.