Just 15 miles from Cologne and also on the mighty Rhine river is Bonn the former capital of West Germany (1949 to 1990). It is also the birthplace of Ludwig van Beethoven, the famous composer and pianist, who was born in 1770 and moved to Vienna in 1791.
Bonn seems like a small city and the tourist sites are centrally located and near the train station. The Munsterplatz has the statue of Beethoven, the Old Town Hall, and the cathedral (currently closed for construction, May 2, 2019). The medieval Sterntor city gate a block over and Beethoven's house is a few blocks away and easy to find. There is also a promenade along the Rhine River about six blocks from the city center.
It's a nice day trip and an inexpensive, 20 minute train ride from Cologne Hauptbahnhof (central train station) with frequent trains.