We drove up to Cologne last month to see the magnificent cathedral and several of the markets. If you go in December, we’d recommend taking the train, as the entire area near the cathedral is blocked off to vehicles. It took us nearly 3 hours from Kaiserslautern due to the heavy traffic once we got off the Autobahn near Cologne. The train time had originally been equal to drive time, not including traffic delays.
The market furthest away from the cathedral that we walked to was a couple of blocks beyond the crowded shopping streets. The market itself was also wall to wall people, but we really liked a vendor selling foot-high carved wood trees with matching colored ornaments already hung and included. The trees were painted in various colors. There is surely much more to see in Cologne, but we were ready to head home after dinner and a short stroll along the river promenade.