I think the just strolling around in Cologne will take easily 1.5 days and if I had some free time, I'd go to Haus der Geschichte in Bonn.

The Haus der Geschichte as part of the Museumsmeile always unfolds nem pages of Germany's recent past.
Approximately 7,000 original exhibits ranging from a railway saloon car of the federal chancellors to an original cinema from the 1950s up to parts of the Berlin wall, Honecker’s warrant of arrest and the first Green Card for a foreign IT specialist are displayed here. Historical film and tonal documents, as well as numerous media stations invite the young and the old for an active deal with their own history.

About one million visitors come to see the various exhibitions of the Haus der Geshichte (House of History) per year.

Free admission!