Cologne has become one of my favorite German cities since we went there last year for the Christmas market. Looking forward to visiting it in summer as in winter it's very cold which makes relaxing and walking around very difficult. The Christmas market was amazing though, set up right next to the Dom.
A trip to Germany around Christmas time isn't complete until you have visited a Christmas market, and Cologne has plenty of these to go around. There were so many choices of food, from Bratwurst, Raclette, Mulled wine, and countless others, that even eating every 20 minutes for the whole day and you would not be able to try everything there is. Definitely a sight to be seen if you have never experienced a German Christmas market.
The view from the top of the South Tower of the Cologne Cathedral was inspiring. While it was completely worth it, the trek to the top of the 600+ stairs in a narrow spiral staircase to reach the top was truly challenging.