The Rhine river valley provides an amazing opportunity for a terrific vacation ( I lived in Bonn for a awhile back in the day). Not the question asked by the OP, but I'd recommend flying into Basel and renting a car. Drive the length of the river along its banks as much as possible. Stop for overnights along the way in the various towns of interest. Remagen (
https://en.wikipedia.org/wiki/Battle_of_Remagen) for example, you can still see bullet holes in the original bridge columns and artillery impacts along the German tunnels in the cliffs.
Perhaps a castle-based hotel or two as you slowly head north. The cost of the cruise for two would pay for some very nice accomodations and meals/wine over the course of a week.I'd probably turn in the car and fly home from Cologne, unless you've never been to Amsterdam and really want to drive on the autobahn while its still got some "no limit" zones.
Added benefit........hit the Nuerbergring less than as hour from Cologne before turning in the rental.