Hi All - Going with friends this year on the AmaWaterways AmaMagna. (Vilshofen to Budapest)
We are looking at the various excursions and while does not look like any would be a mistake, would be interested in the thoughts of those who have already completed. We are open to any including the biking excursions.
For Passau we are pretty sure that we would do the 'three cities walking tour, and think we are set on Vienna during the day (Imperial Vienna Tour and Schönbrunn Pala
Here's a link to my post-cruise review. We did the full Grand Danube, so Vilshofen to Budapest is the first half.
I was surprised @CastleCritic didn't post a link to his excellent day by day review, but then realized he did the second half Budapest to Bucharest. But I'll link it below also since you are going on AmaMagna.
To answer your questions directly:
Passau - If you've never been there before, can't go wrong with the city walk. It's a neat place to see and tour.