Must see in Vienna

See your other post:
https://community.ricksteves.com/travel-forum/tours/post-tour-in-vienna

The RS guide shows a list of ranked "attractions" for Vienna.

It depends on what you are already seeing on the RS tour and what your interests are.

Pick from the list what appeals most to you:
https://www.wien.info/en/recommendations/vienna-in-three-days

Depends on what your passions are.

For us, it was the Spanish Riding School. We couldn't afford the show but we COULD afford watching the training sessions that were 2 hours long. That was a dream for me!

My favorite museum was the Kunsthistoriches:
https://www.wien.info/en/sightseeing/museums-exhibitions/top/kunsthistorisches-museum

I loved wandering the Naschmarkt:
http://www.naschmarkt-vienna.com/

For an inexpensive self-guided Ringstrasse sightseeing tour, download the RickSteves audioguide and buy an €8.00 24-hour transit pass; then "hop-on hop-off" Trams 1 and 2:

https://www.ricksteves.com/watch-read-listen/audio/audio-tours/austria-germany
http://podcasts.ricksteves.com/walkingtours/ViennasRingstrasseTramTour.mp3
https://shop.wienerlinien.at/index.php/product/3/show/0/0/0/0

My idea of must-see in Vienna is probably quite different from anyone else's--my first visit there was all about The Third Man; walking tour, museum visit, finding the Harry Lime doorway on Schreyvogelgasse, finding his fictitious apartment at 15 Stiftgasse, riding the Riesenrad, lunch at Café Mozart and seeing the movie at the theatre next to my hotel...only thing I missed was the Zentralfriedhof.

Generally speaking, if I wasn't a Third Man geek I'd probably check out a museum or two (Vienna is crammed full of art from my most favorite era, the Secession), ride the Riesenrad in the Prater, joy ride on the tram that circles the Ringstrasse, explore the Graben and Stephansplatz area and have dinner at Cafe Frauenhuber. If it was a Saturday I'd go to the flohmarkt at the Naschmarkt, and admire the Majolica haus designed by Otto Wagner.

I'm a fan of the Third Man movie as well. I have only just found out that there is a Third Man museum which I hope to visit when I am in Vienna in May.
Vienna was the first European city I visited in 1979 and I have fond memories of the visit. High on my list of priorities then was a trip on the famous Prater wheel. It was bitterly cold the night I visited the fairground and there weren't a lot of people around. Every time my cabin reached ground level I was expecting at any moment for the door to swing open and Harry Lime aka Orson Welles to step in and join me.

The Third Man Museum has very limited opening times. Be sure to check if planning to visit:
http://www.3mpc.net

They still do the sewer tours, right? Also, Schloss Belvedere if not already in your plans. Go in the morning and start at the top of the hill then work your way down to Lower Belvedere. Exit here and lunch at Salm Bräu.