Specific questions related to Salzburg/Vienna/Munich and undetermined driving itinerary

First of all, May 1 is a public holiday in Germany and Austria. It is the European Labor Day (May Day).

As such, you will not be restricted to traveling on a Bayern Ticket after 9 am. You can take the first available S-Bahn train. If you take the S-8, you can short cut going to the Hauptbahnhof, and change to the Meridian train to Salzburg at Ostbahnhof. Cost for 2 will be 31 EUR bought from a train ticket vending machine, or pay 2 EUR more if you buy at a ticket window. Write your names on the back of the ticket in ink. You must use the Meridian regional train between Munich and Salzburg, no EC or RJ express trains. The ticket is good for any regional trains (or S-Bahns, U-bahns, public buses) in Bavaria until 3 am the following day, so no worries on that front. Valid to Salzburg Hauptbahnhof, but not local transport in Salzburg.

To go to Vienna, you can use the frequent Westbahn trains that have a low anyday price. You can just board the train, sit down and pay the conductor when he/she comes around. They alternate between trains that go to Vienna Westbahnhof, and trains that got to Vienna Haupbahnhof as well as other underground stations in Vienna.

Returning to Munich, I'd get a nonrefundable discount ticket now for a direct express RJ or EC train.

Thank you so much; this is very helpful.

Just so I'm clear, you're saying we can find a vending machine (or ticket window) at the airport, buy a ticket for the S-8 train, which will reach Ostbahnhof station before Hauptbahnhof, so we should get off at Ostbahnhof and board a Meridian train to Salzburg. Our ticket takes us straight through, so we need only purchase that first one, which should be 31 or 33 EUR total?

And regarding the Salzburg to Vienna train, is it just the ones bound for Hauptbahnhof that continue on to other underground stations? Is there a way to know if Rathaus is one of them?

Yes, one ticket gets you to Salzburg. Ticket machines at the airport train station will be obvious as you'll see lots of others using them. Just press the British flag icon to proceed in English.

Here is the Westbahn site showing their route map and links to the time table.
https://westbahn.at/en/timetable-info/

Rathaus is on the U2 line, so you would have to transfer from either Westbahnhof or one of the other stations on the Hauptbahnhof, Rennweg, Mitte, Praterstern line. Only Praterstern has a direct connection to the U2 line, so a bit of backtracking is required.

If you catch an early train to Salzburg you'll have a day and a half before you leave for Vienna. Provided you're up to it I'd drop my luggage at the hotel (they may not have a room ready, but they should hold your luggage) and go up to the castle. You can eat up there on the walls and look out over the city and tour the exhibits. It's also a good way to get an aerial view of the city. I find that helps to give me a sense of direction for later wandering. (Salzburg is an easy town to walk around, even if you walk up to the castle instead of taking the incline rail.)

You could then do dinner in the old town, and plan on a fairly quiet day walking down the river and through the town on the 2nd. The Dom, the University, and walking the back streets can easily take a day. For concert info there's a tourist info place just off the residence plaza; check with them. (Look for the large "Easter Egg".) If the weather's good half the city will be walking down by the river if they can. Likewise, there should be no problem finding musicians to listen to; especially around the Dom.

In Vienna you should be able to use the tram to get to Schonnbrun, but I always use the U-Bahn if possible. It's faster. Two days isn't a lot of time in Vienna. I'd pass on the river ride. From where you're staying it's a fairly easy walk to the museum quarter or the inner city, and since that's in the opposite direction as the Schonnbrun I'd plan on that for the other day you'll have. If you like art or history you can easily spend half a day in the Hofburg or the Kunst (art) Museums. Or the Opera House if you want an overdose of Art Deco. Take time to sit and have coffee and a pastry.

From the Munich Haptbahnhof it's less than 10 minutes walk to Marienplatz, unless you stop along the way. If you get moving early (8-9am) you can watch all the city workers going to work. I'd stop by the victual market and grab some supplies and then wander over to the English Garden, and walk back along the river to the Deutsches Museum. Or you could tour the Residence. There are a number of good beer gardens in walking distance, but if the weather's good I'll recommend the one in the English Garden. If not, Augustiner Brau has several around the old town, and I prefer them to the Hofbrauhaus or Lowenbrau.

Sam, thanks so much for the clarification, and KGC, many thanks for all the great suggestions!

In Vienna the 13A bus route starts at the Hauptbahnhof, where your train will arrive, and goes all the way to the area where your AirBNB is. This will be more convenient than lugging your luggage around U-Bahn stations. The route 2 tram will take you to Prater via the Ring, or if you want to get there faster you can get the U2 U-Bahn line from Rathaus.

The bit of the Danube around Vienna itself is not scenic at all. The nicest bit is the so-called Wachau between Krems an der Donau and Melk, but that's a full day trip from Vienna so you won't have time.

Thank you, Philip. Do you think our best bet would be to get a 72-hr public transport pass for 17EUR, rather than fool around with individual tickets? (My husband would qualify for the senior discount, but we don't generally bother trying to figure stuff like that out when the savings are minimal.) Looks like we could also get a Vienna card for 26EUR, which would include the transportation, and various discounts which we might or might not use, but looks like some of them apply to Prater and Schonbrunn, and if we didn't use them, it wouldn't be a big loss.

1. May 1 is a bank holiday. You may be able to use a pass, ask. A Bayern pass, or similar, works for Salzburg station with no additional ticket needed.

2. Salzburg is a great city to just explore. You can walk around the old center, Mirabell Garden and take the funicular up to the castle in one full day. In the evening you can eat at the Augustiner Brewery. When walking back in the evening, view the old center from the center of one of the walking bridges across the river. That view, with the city lit up, is one of my favorite in the world.

3. In Munich, I generally prefer to be in or very near the center. I only stay near the airport if the flight is so early I'd have to leave the hotel in the middle of the night to get there.

4. English garden is a good choice after Marienplatz. If the weather is nice, go to a beer garden there. Have a back up plan if it's not nice. I love the Alte and Neue Art museums (Modern is beyond my tastes). Another option is Nymphenburg Palace.

Should have added the walk from Salzburg station to the center is a fairly good hike. I'd guess more than a mile. I've walked it more than once but you might prefer to taxi.

Salzburg is a beautiful and compact town to walk in. We walked everywhere we needed to go with the exception of Stiegl Brewery where we took the bus, but then just walked back because we realized it was a very short distance away. The square around Salzburg Museum is charming, as is the view from the river. Definitely make time for the old town, the fortress, Mozartplatz Square and Salzburg Cathedral/ St. Peter’s Abbey. The Mirabell Palace and Garden are nice if you have time (just don’t tell him it features in the movie. You’ll tend to happen across sites from the movie by chance anyway; Salzburg is not very big).

It's up to you to decide whether the 72 hour pass is worthwhile. Regarding the Vienna Card, make a careful list of all the things that you are likely to use (and not just for the sake of getting the discount) and see if it works out financially. I must say that even with the Vienna Card, Vienna is one of the most expensive cities in Europe in terms of museum entry fees.

The Neue Pinakothek in Munich is closed for several years for renovations.

Thank you, Brad and Roubrat. We are good with the walking in Salzburg; just decided to get a hotel near the station so we'd be right there upon arrival, when we'll be tired, and upon departure. The walking will be in between , after we've been able to rest!