Regarding Kolsch, which is not an especially complex flavor, very as we say in the US today, "blonde beer": It's always served by gravity from a small keg on the counter. So it's nice to watch the serving process and keg changes. Gaffel is considered a touristy location, near the Dom, but the serving room and the dining room actually have nice aura of authenticity. It's my preference. Note the "wreath" serving trays for a table-full of customers. I think I also went to Peters or Sion, partly because Lonely Planet Germany said they had potato pancakes on Thursdays. This is by no means a comprehensive list. Note that Kolsch is often the single brand available in a bar with the same name. See if multi-tasting has become available in modern times?
As noted by implication, Cologne is NOT a scenic Rhine river-tour location. I thought the local tourist boat was a waste of time. When we were in Boppard, there was a nice wine garden (mostly paved) right across the street from the train station.
If you really like beer, that might lean towards a Duesseldorf visit, where they have a darker beer called Altbier, served in .3 liter glasses. Lonely Planet says not to speak the word "Kolsch" while in Duesseldorf! Serious drinkers in Duesseldorf (and there are a lot of them) alternate Altbier with Killepisch, an herbal liqueur which is actually kind of good, much more palatable than Jagermeister. (An American altbier is made by Long Trail, in Vermont, and widely sold in the northeast, at least.)
You have a good list of other visits. It's not close by, but if your whole trip lacks Fachwerke, and you can't make time for Quedlinburg, you could consider Monschau, which is a touristier version of Quedlinburg, without the Church treasury. Nothing else there, except maybe some hiking.
You should look up the museums (many) in Cologne to plan your time. Even if you happen to dislike miles of oils on the wall, places like the MAK and Kolumba (both HBF walkable) and the Medieval (Schnütgen) are a great antidote for that. The Roman museum (HBF walkable) is quite famous, and not dry or boring, and Cologne is an important Roman location, with an underground monument with tunnels, the Praetorium (Google says currently closed, but Google isn't reliable on that.) There is a medieval Mikveh that you used to be able to sign out the key for with your passport, don't know today. The small "old town" is one of the less authentic ones, due to war damage. The restaurants with riverfront terraces don't look promising to me. I'd stick with the Kolsch brewery outlets.
Note for those interested in "The Paths of the Goddess", Rome, or travelers of faith, Cologne is very closely associated with St. Ursula and her horde of ... (word choice for internet reasons ... ) "maidens."