Vienna-3 nights

Have you looked at Austria in the Explore Europe section of the site?

https://www.ricksteves.com/europe/austria

Click on Vienna, then At a Glance

My favorite sightseeing activities for my 4 days spent in Vienna last fall were:

Schönbrunn Palace
Hofburg Palace, Imperial Apartments, Imperial Silver Collection
Kunsthistorisches Museum
Tour of the Spanish Riding School
Wandering the Naschmarkt

Self-guided tours using the Rick Steves audio app and guidebook:
Vienna City Walk: St Stephen's Cathedral, Old Town, etc.
Vienna Ringstrasse Tram Tour

https://www.ricksteves.com/watch-read-listen/audio/audio-europe

I also really enjoyed seeing the Opera House, which I did by attending a ballet.

I have to say that the Vienna Pass is one of the rare visitor passes that justifies the cost - especially if your friends are art buffs.

https://www.viennapass.com/vienna-attractions/

Our favorite restaurants (2015) were Fromme Helene and Marienhof.

https://www.frommehelene.at/index.php?id=1&L=1&amp%3BL=1%5C%5C%5C%5C%5C%5C%5C%27

http://www.restaurant-marienhof.at/restaurant/

Also, the Opera is as good as it gets - if they like opera. If they decide to attend a performance, I suggest that they splurge on the best seats they can afford.

Spanish Riding School. We went to a practice and it was wonderful.
For lunch and pastries, Demel is excellent!

https://www.wien.info/en/recommendations/vienna-in-three-days

For a quick bite, Trzesniewski's (open face sandwiches and a tiny beer).

For schnitzel and other traditional dishes; nice atmosphere, excellent service: Figlmüller.

Both are easily located near Stephensplatz (and perhaps listed in Rick's books).