Well, that entirely depends on your focus, doesn't it?

But there is at least one building one has to see in Cologne, since it is its landmark: the cathedral (in German: Dom). There are a few other interesting buildings, but you can easily make a list of those on wikipedia.

I would plan a longer stay and use it to visit different cities (and countries - France ist not far away)

Not sure when you're planning on visiting, but we had the good fortune to spend last New Year's in Berlin. Highly recommended!