The Green route is a 90 minute hop-on-hop-off tour of the city that starts at the Votivkirche and heads out north out of the city to the wine growing suburb of Grinzing.
From Grinzing the bus takes the scenic road up to the Kahlenberg from where (assuming it's not shrouded in fog as it was on the day I went) there are views over the whole of the city.
From the Kahlenberg the bus returns to the city centre.
Multi-lingual pre-recorded commentary tells you about the sights as you travel round