Linz, Austria’s third-largest city, lies on both banks of the Danube and is an important industrial town and busy river port. An optional full-day excursion to Salzburg takes in the popular Sound of Music sights. Passau, ideally located at the confluence of the Rivers Inn, Ilz, and Danube, is a city of graceful arcades and cobblestone streets. St. Stephen’s Cathedral houses the world’s largest church organ with more than 17,000 pipes. (B,L,D)