Hello, Vienna Christmas Market expert here.
I suggest that you look at previous posts I have made, breaking down all of the markets. Even though these are a few years old, I promise you that nothing changes. I make a point to go to every Christmas Market in Vienna every year. We additionally try to go to another town every year for other markets:
https://community.ricksteves.com/travel-forum/austria/a-local-s-take-on-15-christmas-markets-in-vienna-2019
https://community.ricksteves.com/travel-forum/austria/a-local-s-take-on-vienna-christmas-markets-2022-edition
As you will read, my favorite Christmas markets are Karlsplatz, Spittelberg and Altes AKH.
For a day trip to Salzburg, I recommend you visit the main Christmas market and the fortress. Also wander the streets in the old city center. It will be cold.