Koln/Bon in 4 days

Rick's article on Koln describes the gothic cathedral, Roman history museum, and modern art museum that are all right at the center of the city. You can daytrip to the Rhine River Valley, for instance 2.5 hours to St. Goar by the slower regional train that runs along the river. Or to the even smaller and cuter Mosel River Valley, for instance 1.5 - 2 hours by train to Cochem with a connection at Koblenz. For ferry schedules on both rivers, see www.k-d.com/en/home. I'm not familiar with the modern city of Bonn, but understand that you can see Beethoven's birth house.

Laura has sent you to the Middle Rhine Valley, the most dramatic part of the Rhine which lies south of Koblenz; St. Goar photo, map, and links to towns there are here.

It's a fairly long trip south if you see the whole section but worth the trip if you have ALL day (go as far south as Bingen if you can; use the RE train between Cologne and Koblenz instead of the slow MRB train to get through this northerly section faster.) Laura has provided a link to the KD cruise boat line - which you might want to use from Bingen north to St. Goar (a ride of 1.5 hours) if you have the time (and the extra €.) It's a nice way to enjoy the scenery if you can make the somewhat skimpy boat schedule work with the rest of your plans for the day. St. Goar is a good stop if you wish to see Rheinfels Castle (ruins and museum.) If not, I'd visit some of the other towns instead (Oberwesel, Bacharach, and Boppard - nice chairlift ride in Boppard - have a larger number of very attractive old-world structures.) Braubach (east bank of the river) is home to Marksburg, a never-destroyed castle with furnished interior that offers guided tours at a reasonable price. You can get there in 10 minutes by train from Koblenz - or from St. Goar you can FERRY across to St. Goarshausen (this is a real ferry that runs all day long) and take the train north to Braubach from St. G'hausen (20 min.)

Marksburg

But there are some much closer outings you might like too. Aachen is a fine city. Linz am Rhein is perhaps the most attractive old-world town on the Rhine and only 45 min. south of Bonn, 60 min. from Cologne:
Linz am Rhein

From Linz, you can FERRY (not cruise boat) across to Remagen and check out the WW II museum there at the site of the former Ludendorf bridge - great little museum:

Peace Museum

It is also possible to use the KD cruise boat from Linz or Remagen back to Bonn, a nice trip if not as dramatic as the Middle Rhine cruise.

Bad Münstereifel is a very attractive little town in the mountains west of Bonn and accessible by train from there:

B-M 1
B-M 2
In Cologne itself the Documentation Center is one of numerous museums that might interest you.
NS Doc Center

Thank you for these great suggestions. I have printed this out and it sounds wonderful.