My choices are an overnight train to Cologne and staying there 3 nights and focusing my attention on Cologne. Never been to either so I wanted to see what you think of those cities Or staying 2 nights/days in Mainz and 2 night/days in Cologne. Also is there buses that go from Mainz to Cologne? Thanks!
Mainz is nice if you are in the area (because you have a flight from FRA for instance). But I don't think that it is worth the 200km or so from Köln (Cologne), unless you have a specific reason to go there.
Why don't you combine Köln with Düsseldorf for instance?
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Cologne is a great city. There are Roman ruins there - you can actually walk in the old Roman sewers under the streets. There is a great Roman museum there. Of course there is the Dom, which is spectacular. And there is the chocolate museum (and shop), which I really enjoyed. At Christmas, there are at least 5 Christmas markets in the city.
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