Rhine river day cruise starting and ending in Cologne

A day cruise from K-D, starting and ending in Cologne, could only go as far "up" the river (to the southeast, against the current) as Bad Hönningen before you would have to come back. You would not get as far as Koblenz.'

There is only boat out of Cologne (and one ending there) per day. That's not the most popular segment of the river.

See the K-D schedule, here.

Best plan is to take the train up river to Bingen (Rhein), then board a downriver boat. After about Boppard, you could get off and train back to Cologne, or ride all the way to Cologne. There is a change of boats between Koblenz and Cologne. The lower trip is not as scenic, although the abutments of the old Remagen Bridge are interesting historically.

There is a harbor-cruise kind of local experience, maybe 90 minutes from Cologne, which I have done. There are a tiny number of notable buildings, but it's a sterile experience. As noted, this is a forgettable section of the river. The only area worth taking the KD boat is Bingen to Boppard, in that direction. Cologne is way, way too far from that section to boat from. All these boats are seasonal, BTW.

Another (crowded) option to watch for is Rhine in Flames, a one-night annual event, not in Cologne.

On a pleasant day, it's actually very nice to walk over one bridge in Cologne and back on another, with museum options along the way. I also like to approximate the (right bank) view of the Turner painting of Cologne that's in the Frick Collection.