We actually did this at the end of our visit to Vienna (we would normally do a tour at the beginning). It was 7 € each which includes headphones for the commentary (available in many languages) and a screen showing the sites you were looking for. It followed a 'ring' road and luckily showed us many beautiful buildings and parks we hadn't visited e.g the parliament building. You can get off and on if you wish but we just did the round then hopped onto the underground.
If you own or manage Vienna Ring Tram, register now for free tools to enhance your listing, attract new reviews, and respond to reviewers.