Trier (founded by the Romans - oldest town in Germany). Very fun to stroll the streets or peek into the cathedrals. I saw the robe of Jesus here in 2012. www.RiverCruiseGuru.com for free assistance with your river cruise.
My grandmothers synagogue - Neue Synagoge (New Synagogue) in Essen, Germany. Now the Alte Synagoge (Old Synagogue). It's destruction - Kristallnacht 9–10 November 1938. Miraculously the exterior survived.