Why locals love it
Vienna is one of very few citys which have a thermal spa in the neighborhood. And the Viennese people do appreciate this fact. The thermal spa Vienna is easily accesible via tram line 67 and was just renovated to fullfill the requirements of a health and wellness resort in the 21st century.
Why you should visit it
Visiting a thermal spa is quite an unusual thing to do on a city trip. Maybe that's a reason to go there. You can spend a day in the city and do all the typical things you do in a city (e.g. shopping, museum, cafés, ...) and then relax in the thermal spa in the evening. That's something only Vienna offers.
The spa is open till 22:00 o'clock. Take a 3-hour-ticket and spent the evening in the cozy pools there.