Just a normal day, a normal ride on the tram, during the less busy hours of the day.
If you want to see the main sites in Vienna, but don’t feel like forking out for a tourist bus, you can get a day-pass (unless you already have one with Wienerlinien, then you don’t need to do anything more) and take the the D-line from one end to the next. It passes most of the important things in Vienna, and goes at a leisurely pace. You’ll pass the important theaters, the parliament, most of the museums, the opera house, Belvedere castle, Fernwärme Wien (a building designed by Hundertwasser)… Really, if you want a hop-on hop-off line that doesn’t have an extra price tag, and that is also not full of tourists – it’s the perfect choice. 😉