I have only been to Cologne (Köln) once so far and was disappointed by my visit because I love history. Cologne is a very modern industrial city and does not have the same history as other cities in Europe. This is because of an industrial boom in recent decades and because of the mass destruction it faced during WWII. It is not a very popular destination for tourists, which is nice because it is less busy, since there is only one real tourist attraction in the city.
The Köln Cathedral is ginormous. I think it is still to date the tallest Cathedral that I have visited- and I have been to many. The scale of the building is ridiculous no matter what angle you look at it from. The outside is an extremely imposing dark gothic facade with incredibly detailed decoration on every available space. The ceilings on the inside loom so far above your head that they look like a CGI scene from Lord of the Rings. The interior may be considered bare in comparison with other cathedrals, but it there is more wall space available for filling than in any other cathedral I have seen. It is worth visiting just to appreciate the feat of architecture that it is.
Up, Up, Up
The thing that you REALLY should do when you visit the cathedral, is climb one of the spires. Try not to do it on a hot day because it is a very very very very very very very very very very very very long way to the top. Skip leg day and just climb this baby instead. If you manage to make it up the 533 steps and 475 foot climb, you will be rewarded with views of the interior of the bell tower and a panorama of the city. Not going to lie, I was too busy shitting a brick to take pictures of or appreciate the view by the time I made it to the top. It was very high and very windy so I caught my breath and headed back down. As a result, I have no photos from the top of the spire. It was still an amazing experience and I am glad that I did it even though I am not crazy about heights.