Mozart's birthplace, a fortress you can see from everywhere, The Sound of Music locations, and arguably the best Schnitzel in the world. Just over 1.5h by train from Munich.
Munich to Salzburg is a straightforward regional/EC train ride from München Hauptbahnhof to Salzburg Hauptbahnhof. Trains run roughly hourly, journey time is around 1h30–1h45. As you approach Salzburg, the Alps start to appear out the window. The Austrian border is seamless (Schengen zone — no passport check). Salzburg Hbf is a ~15 min walk or short bus ride from the Altstadt.
Tickets: Book via the DB Navigator or ÖBB app. No seat reservation required for regional services.
Regional trains don't require reservations, but check the DB/ÖBB timetable a day or two ahead to lock in a departure that leaves enough time on the ground. Group tickets (Bayern-Ticket or similar) can be cheaper than four separate fares — worth checking before buying.