We want to take a day trip to Salzburg from Munich in October (Fri, Sat, or Sun)
Which day's best?
What train?
What to do?
Help!
What to do is up to you.
Going on a Saturday or Sunday, you can get an early start using a Bayern Ticket, which is 23 euro for the first person and 5 euro for each additional person for up to 5 traveling together. You must travel on the local Meridian trains, not the Railjet or EC express trains. On week days (Friday) the ticket is only good after 9 am, but no time restrictions on weekends. The one ticket will cover you for the rest of the day including your return and transfers on the Munich transit system, but not local transit in Salzburg.
See the schedule at www.bahn.com
More about the Bayern Ticket at
https://www.bahn.com/i/view/USA/en/prices/germany-regional/bavaria-ticket.shtml
Most people would use something like this.
The Bayern-Ticket is a hop on/off ticket for regional trains, so you can stop anywhere as long as you like. There are not really a lot of noteworthy things near the main line between Salzburg and Munich. Except for Herrenchiemsee Palace. You get off for a couple of hours at Prien, walk or take the steam streetcar to the harbor, then take a boat over to the island and walk to the palace. Herrenchiemsee is a partial replica of Versailles bullt by King Ludwig, who was fascinated by French King Louis.
A walk from the train station up to the castle and back with a cafe stop and lunch would pretty much fill a day for us. Take in the Mirabell Garten with views of the castle and city. Walk along the Salzach River with views towards town. Visit St. Peter's and its grave yard (Petersfriedhof), and the Franziskanerkirche (Franciscan Church). The views from the walk up to the castle and from the castle are great. There is museum in the castle with some areas still being excavated. Do a little shopping in the old town, if not on Sunday.
We are not big museum or exhibit folks, so your tastes may be different.
Salzburg website has a pretty extensive list of sights.
Take in the Mirabell Garten with views of the castle and city
Mirabell, by the way, is the location of the steps where the children sang do-re-mi in front of the Pegasus statue in Sound of Music, one of the many SOM venues you can easily access without a tour. The arbor they ran through is also on the property.