Typically, I would have researched this myself, but I have been out of the country for a couple weeks and just learned I will have this free time. (I even have Rick Steve's Germany 2013 at home on my shelf! Oh well!) I was in Germany this summer but most of that time was spent in Bavaria with some time in Berlin.
I will have a free day in Koeln to explore. What would you say I must see? I definitely want to see the famous cathedral. What else?
I will have a free day next Monday in Dusseldorf to myself from my work trip. I will be staying in Dusseldorf on Sunday night but I will not be free until after 7 p.m. that night-so I am mostly looking for ideas for Monday. I will catch a train back to my temporary home base in the Netherlands around 6 or 7 p.m.
I am happy to walk all day long. I love outdoor sites (and this weather is beautiful for a person from the cold northern part of the US!), history, art, and am truly open to just about any suggestion!