Cologne to Baden Baden

That's the first time I've heard of that combination.

Let's look at it.

If you are going in that direction you are going upstream and the trip will take quite a lot longer than the other way because of the current. Most of the river is not particularly scenic other than the area where the castles is, so the only long distance passenger ships traversing the Rhine for its length are the big tour company ones such as Viking. But their package normally is Amsterdam to and from Basel, not between intermediary ports. Strasbourg would be the nearest port, and then 2 trains. Viking gets to Strasbourg in 4 days.

Might be worth asking them if they take part trip passengers.

Train - just over 2 hours with one easy change in Mannheim.

I don't think so. There really is not too much to see on the river other than between Koblenz and Mainz. I know there is a beautiful oil refinery just south of Cologne. There are the old bridge abutments of the Ludendorff bridge at Remagen still standing where the 9th Armored Division crossed the Rhine in WWII before it could be destroyed by the Germans. One side is a German-American Friendship memorial.

The real scenic parts of the river are from Boppard to Ruedesheim. You could catch the classic steamship of the K-D line at Koblenz at 9:00 am and ride that all the way to Ruedesheim, arriving at 3:15 pm. Spend the night in Ruedesheim, and get a train to Baden-Baden in the morning. Better would be to take a train to Braubach and visit Marksburg castle, spend the night and get the steamboat at 10:05 the next morning.

So that would take 3 days and 2 nights to get to Baden-Baden, same as an overnight cruise ship would take. The river south (upstream) of Ruedesheim gets very industrial.

"There are the old bridge abutments of the Ludendorff bridge at Remagen"

You will find this sight in the "other" popular scenic cruise segment between Bonn and Linz. This segment plays 2nd fiddle to the most popular Boppard-St. Goar-Rüdesheim segment Sam suggests (which has 5boats in each direction to/from Boppard,) but it is a nice area worth cruising as well if you have the time. It has 4 boats per day (see p. 7 of KD pdf below for Bonn-Linz schedule) and is well patronized.

https://www.k-d.com/fileadmin/schiffstouren/KD_Fahrplan_international_2019.pdf

Linz is a very attractive old-world town. Remagen (just to the north) is nice as well. There is a worthwhile WW II museum in the old bridge abutments in Remagen:
http://www.bruecke-remagen.de/index_en.htm

I would strongly suggest that you spend at least a night in the Boppard - Rüdesheim area. This part of the Rhine is a UNESCO World Heritage site, and the old-world towns here are delightful in a way that Cologne will never be. Also, it's a shame to just cruise past the castles without touring one or two. Two are easily accessible. Marksburg Castle Braubach (south of Koblenz on the east bank) is an intact medieval castle with tours in English at certain hours. Rheinfels in St. Goar is a might set of ruins with a worthwhile museum. Last June we stayed at the Rheinhotel St. Goar, right near the KD boat dock in St. Goar, last June and enjoyed this view across the river

Ah yes, I do recall reading that but had forgotten. Like that classsic side-wheeler we used for a dinner cruise in Budapest. You could see through the viewing window twin diesels with electro-motive drive.

Thank you all. We appreciate all your thoughts and suggestions. We are still very much in the "just thinking" stage but you have given us ideas we would not have come up with on our own.