To get one of the best views over the city, the Kahlenberg is a great choice, especially if you combine it with a small hike. Even though a direct bus connection is available for €2 it is much more rewarding if you take the hour-long walk and watch the view over Vienna unfold a little bit more with every step up.
Either way, by bus or on foot, what you get at the top is a great view over the whole city of Vienna. There’s a panorama terrace with a café and a restaurant, but be vary – they tend to be overpriced, and the restaurant is not quite amongst Vienna’s highly recommended choices.
Most of the time it is quite windy, so chances are that after a couple of minutes of enjoying the view, you will want to make your way back down.