Rhine River Cruise on Emerald

Honestly, it would depend on the time in town and the "free" tour options from the cruise line.

I'm assuming Basel is the start and Amsterdam the finish, so with those, you have the option to extend time there and/or do pre/post cruise regional explorations relatively easily. Both places have a ton of options to fill some extra non-cruise days, and for those two, I would start a single thread for each in their respective Switzerland and Netherlands forums.

For most spots in the middle, the cathedrals would be my DON'T MISS attractions, but also likely included - for sure - in the ship tour options for places like Freiburg, Strasburg, Cologne. Koblenz has its popular Deutsches Eck (German corner?), and is also near castles (worth a google to see which are open and nearby).

Maybe worth pointing out, the boat will not dock in Freiburg or Heidelberg.

For Freiburg, the boat will dock in Breisach likely, about a half hour or so from Freiburg.

For Heidelberg, the boat stops somewhere near Mannheim, again, about a half hour away.

If the tour offers excursions, that might be wise for those two towns, since the towns they dock at have limited appeal.

We were on Emerald from Amsterdam to Basel in May. Here's my Trip Report if you're interested. Note that the docking port in Basel is in France about a half hour tram ride and then a 10 minute walk, unless you're using Emerald's pick-up service.
https://community.ricksteves.com/travel-forum/trip-reports/trip-report-amsterdam-to-zurich-by-boat-and-train

Koblenz: On the whole, the town itself is somewhat "meh" IMHO; the small old-town zone near the docks is nice for a meal/drinks; the "Deutsches Eck" is right there too, but really, it's not something you'll write home about... you walk out onto the concrete platform at the confluence of the Rhine and Mosel Rivers, admire the statue of Emperor Wilhelm, and that's that. It may have some sentimental importance to Germans, but there are better ways to spend your time; I think the cruise companies suggest it because it's easy to access:

https://upload.wikimedia.org/wikipedia/commons/thumb/9/96/Koblenz_-_Panorama_von_Festung_Ehrenbreitstein.jpg/1200px-Koblenz_-_Panorama_von_Festung_Ehrenbreitstein.jpg

The gondola ride to Ehrenbreitstein Fortress is probably your best bet in Koblenz - it whisks you out of town and across the river to a Prussian-era fortress (1800's) on the opposite riverbank. A couple of museums are located inside the fortress.

https://www.seilbahn-koblenz.de/homepage.html

Marksburg Castle - a medieval castle which has stood there for some 800-900 years - is by far the most interesting outing from Koblenz. It's in the town of Braubach, which is just south of Koblenz. You will pass it as your ship heads north.

https://img.fotocommunity.com/marksburg-braubach-bc203e81-d606-4f2f-bb8d-17e275e8ba88.jpg?height=1080

You can take a tour in English at 1 pm and 4 pm:

https://www.marksburg.de/en/circuit/#

If your tour does not offer this outing - or if you just want to do it on your own - I suggest hiring a taxi to take you there - or go by train to Braubach and then you walk up to the castle (25-30 minutes, steep path.) The train leaves from Koblenz City Center station (Stadtmitte) every hour at :49 after the hour (in September) and takes only 16 minutes to reach Braubach. The train ride costs €6.50 per person, double that for the round trip.

Braubach is a beautiful old-world town. Some pics:

https://farm5.staticflickr.com/4143/4751924826_98980aa49f_o.jpg

https://fotos.schloemp.eu/wp-content/gallery/braubach/002Braubach-Bauernschaenke-Eck-Fritz.JPG

A Weingarten near the river in Braubach: https://www.klinge-gastro.de/weingarten/

Thank you, Russ.