Brilliant view over the city from the top of the Kahlenberg in the 19th district.
Take the 38a bus from Grinzing and just stay on the bus till the end of the line.
Updates
Having taken many friends on this journey over the years, last year I discovered that taking the occasional 38A that goes beyond Kahlenberg to Leopoldsberg (every half hour, maybe?) brings you to a more agreeable destination with even finer views, and in both directions (Vienna and Klosterneuburg).
Is this your business?