Thanks Philipp, this is really helping!
In CGN I will probably want a hotel close to work (not suer where it is yet, but I have a German partner who can help).
Could you recommend a hotel in DUS?

And a question about trains - do you have to book in advance, or just hit the counter?
Sorry if it's a silly question, last time I took a train for actual travel (that is, not with the kids for fun) was in 2001 in India...