Cologne. Travel in Germany and learn fluent German with the Eurolingua Institute http://www.eurolingua.com/german/german-homestays-in-germany
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.