Ball Season
One thing you should not miss when you visit vienna during the carnival season is to attend one of the many glamourous balls in the city, some of them in wonderful elegant locations like the vienna hofburg or the city hall rathaus. the most widely known and elegant balls are the opera ball in the state opera (http://www.staatsoper.at) or the ball of the wiener philharmoniker. many professional guilds have their own ball like e.g. the kaffeesiederball by the vienna coffee house owners. the ball calendar can be found on the pages of the vienna city council
River and Canal tours
 the canal tours are horrible. all you see are trees and industrial buildings. it's a waste of money and time.
ImPulsTanz Festival
In the summer, there is also the ImPulsTanz Festival (http://www.impulstanz.at) for contemporary dance & performance. They are also good if you are interested in dance workshops.