Guten Tag!
We just went from the quay in Cologne and went up the river for about 1.5 hours boat trip. You can buy a meal and drinks on the boat. Didnt go as far as Dusseldorf. It is a bit industrialised, big factories and smoky chimneys. We got back from Recklinghausen last night and flew from Dusseldorf-Weeze with Ryannair from Stansted. Very easy journey, but that is industrialised with lots of canals too. I think the prettiest places are further south. Try Kochem - lovely little old fashioned village by the river. Lovely cafes and wine bars, quaint shops. All very typical German, Pied Piper stuff! Castle on the hill with birds of prey displays. Also, there is a chair lift (I didnt try that, my husband did) that takes you to the top of a mountain with fantastic views. The train service is very good and within an hour or so you can be in the mountains.
Back to Cologne... you can hire bikes which is a simple way of touring the city... great fun!
All in all Cologne is a great place to visit. Have a great time
Chuus