Budapest , where the cruise began is a city not to be missed. The tour of this city was excellent with all of the key attractions visited. Recommend you get there a day early and explore the city.
Vienna, is a wonderful place to visit. Just enjoy the place.
Melk. The monastery in this port is the highlight of the area. You have to see it to believe it.
Linz and Salzburg. The ship docks in Linz and you are bused to Salzburg. Salzburg is a wonderful place but in my opinion it has become over commercialized since my last visit 15 years ago. However it is still worth the time to visit. If you time walk around Linz. You won't be disappointed.
Passau. A great little historic town. The organ recital was not worth the half hour unless you are really into that type of music.