In Cologne, you can relax a couple of hours at Neptunbad (a sauna, spa) in Ehrenfeld - beautiful architecture, very stylish interior. Take a steam bath, one of the hot saunas and dip into the ice cold water afterwards. Rinse and repeat. Note that this is clothing optional and mixed gender (which is the norm for sauna).
Go to Bonn (30 minutes by train) and explore the rich political history of Germany. Cute old town, many museums, visit the 'Haus der Geschichte' (interactive history museum, free).
Many people love the city of Koblenz (ca. 2 hours by train from Cologne), famous for its wine, Roman ruins and stunning views of the Rhine. about 7 years agoAnswered by Klaus W. via WorldNomads.com