I will be traveling with my two sisters who will be 80 next fall and are in good health and mobile. We will likely fly in and out of Frankfurt in order to get a non-stop flight. We are considering Rothenburg on our way to the south. Aside from Munich, which two cities are recommended to highlight. Fussen? Freiburg? Salzburg in Austria?
Are you traveling by train? Just be aware that leaving Rothenburg to Munich, you will have some very tight connection times, like 4 or 5 minutes and needing to use the under-track passageway. You can make if you give a heads up to the conductor. Otherwise, just keep in mind that if you miss a connection, there will be another in 1 hour. Maybe bring a book.
Salzburg sounds good to me. There are a lot of single connection train itineraries back to Frankfurt airport.