Passau is famous for the three rivers joining which is a definite "must see". There are some lovely restaurants (Biergarden style) along the water and the downtown "Altstadt" is very nice, typical Bavarian as well.
Also, there is a big church / cathedral in the center, very beautiful. Can't recall the name.

If you are catholic or if you care for the Pope, the town where he was born and where he grew up is on the way to Passau. Very touristy and in my eyes not worth going there but some people care for it so I feel I want to let you know.