Recommend a Long weekend between Amsterdam and Frankfurt

Cologne has a fairly spiffy cathedral. Adjacent to the hauptbahnhof too.

Speyer does too, but that is south of Frankfurt slightly - do you want to backtrack?

You could go from Amsterdam to Cologne by train. Stay one night in Cologne. Cologne to Koblenz by train. Stay one night in Koblenz. Boat from Koblenz to Mainz and then train to Frankfurt. I don't think you have much time for too many things.

For Cathedrals, certainly Cologne is right on the way, worth at least a stop, a very nice pedestrian shopping area downtown. Aachen might be a worthy sidetrip, one of the oldest Cathedrals in Europe and resting place of Charlemagne. You could look at an overnight if needed, or just pass through.

For kicking back, yeah, the Rhine with St. Goar would be what you are looking for, but take some time to consider the Mosel. Cochem or one of the other small towns would be ideal, bit more laid back.

"What section of the Rhine do you recommend for a scenic boat ride?"

The idea of taking a "Boat from Koblenz to Mainz" would require you to spend 8.5 hours on two separate cruise ships - it's a lot of chugging upstream through some not-so-scenic parts against a strong current.

For a more focused cruise through the most scenic part, cruise only the segment between St. Goar and Bingen. That's the most popular part and is covered by multiple ships per day - and for good reason. And it will cost you only 1.5 - 2.5 hours, depending on which direction you travel. And you'll save a bundle of Euros as well - KD Cruises' standard fare at the dock is about €20, or get a 20% discount by taking the train to your cruise start point (just show the KD kiosk attendant the train ticket you used to get there to get the discount.)

With your extra time on this day, you can stop off in St. Goar, Oberwesel, or Bacharach, some of the nicer old-world destinations.

Where to stay? If you are only interested in Cologne for its cathedral, you could head there for a short while, then catch a train to the more scenic Upper Middle Rhine Valley (Koblenz south to Bingen.) Stay a night in Cologne if the other sights interest you.

Rather than modern Koblenz, consider staying 2-3 nights instead in the old-world town of Boppard (about 15 minute south of Koblenz.)

On Day 2 you could catch a train up the Mosel River Valley to visit Cochem - and perhaps visit either Burg Eltz or Marksburg Castle (in Braubach on the Rhine) on the way (both castles require a short detour.)

Then on Day 3, do your Rhine town visits and cruise.

Thanks so much for the tips - how does this sound?

Friday morning: Train from Amsterdam to Cologne. Have lunch in Cologne and see the cathedral.

Friday afternoon: Train from Cologne to Boppard and lodge there.
Saturday: Train from Boppard to Bingen and then ride a boat from Bingen back to Boppard, stopping off in Bacharach, Oberwesel and/or St. Goar. Arrive back at my hotel in Boppard by the evening.
Sunday morning: Explore Boppard and depart for Mainz.
Sunday afternoon: Have lunch in Mainz and see the cathedral. Depart for Frankfurt and lodge there.
Monday: Day trip from Frankfurt to Speyer Cathedral and possibly Heidelberg, which is very close to it and sounds pretty.
Thanks!

For the 3 cathedrals in Mainz, Speyer and Worms all of which are a bit similar in structure and built around the same time, I do have my favorites. Mainz has the most beautiful interior of all 3, with Worms coming in 2nd because of the addition of the St. Nikolas chapel. Speyer, though the largest of the 3 had a very unimpressive interior. That said, these are the 3 SHUM cities and all 3 are worth a visit for more than just their churches.

If you are looking for interesting churches, I recommend the Union Church in Idstein, St. Elizabeth in Marburg, and the Justinus church in Frankfurt Höchst.
On the Rhein, consider stopping in Eltville and visit Eberbach monastery. Well worth the visit and for me one of the most impressive things to see near the Rhein and the Rheingau region.

I believe the last day for the season for the KD boats on the Rhine is October 20th.

Rent a car. Drive up the Rhine to Koblenz, then up the Mosel Valley to Trier, then back to the Rhine Valley.

I don't know if you're planning to spend any time in Frankfurt, which itself is worthy of 2 full days. It's quite easy to day trip from Frankfurt to Mainz, Wiesbaden, lovely medieval Buedingen.

"I believe the last day for the season for the KD boats on the Rhine is October 20th."
Could anyone provide a link for information on the KD boats? And what KD stands for? My plan is to do it on Saturday, October 19, but I still would like to get an idea of their schedule.

"Frankfurt, which itself is worthy of 2 full days"
I love seeing old art and old churches, and Frankfurt seems so modern. I was planning on going to the Staedel Museum and the Kaiserdom, but would love more recommendations for old art and churches in Frankfurt.

Thanks!

Also, since the KD boat season on the Rhine ends October 20th, will it be too cold on the boat on October 19th? Does anyone have experience with how pleasant it is on Rhine at this time of year?

KD stands for Köln-Düsseldorf, which is the full name of this ship company.
There are usually at least one ship per day after 20 Oct. and if the weather is nice, there will be more. End of Oct. weather can be really nice with crisp, sunny days and blue skies, or cold and rainy. We never know.

I love seeing old art and old churches, and Frankfurt seems so modern. I was planning on going to the Staedel Museum and the Kaiserdom, but would love more recommendations for old art and churches in Frankfurt.

Frankfurt has several medieval churches, the Kaiserdom of course, dating back to late 1300's-middle 1400's, St. Leonhard's built in 1219 and just reopened after 8 years of restoration, Alte Nikolai built in 1290, Deutsche Orden built in 1309, the Karmeliter Cloister built middle 1400's but with 150 m of religious wall paintings. The top church to visit would be the Justinus church in Frankfurt Höchst, consecrated in 850, making it one of the oldest in Germany. Visit Franconoford, which is the archeology site in the middle of the rebuilt new Alt Stadt. Go to the Judengasse, which has the cellars and foundations of the medieval Jewish Ghetto, which was sadly the first walled-in Ghetto in Europe dating back to 1462. This is a very unique museum and the neighboring medieval Jewish cemetery and the Jewish Holocaust Remembrance Wall are a must when you visit Frankfurt, I think.

Could anyone provide a link for information on the KD boats?

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

Schedule is near the bottom. At the time you'll be there there are 4 northbound boats and 4 southbound.

If those times aren't convenient, Bingen-Rüdesheimer also has cruises - 2 per day. What you want is the "Loreleyfahrt", or Loreley Cruise, which will cover the Bingen-St. Goar segment.

https://www.bingen-ruedesheimer.de/en/loreley-cruise

Mainz Churches:

Besides the Dom in Mainz, the Stephanskirche is popular - more popular at Tripadvisor, anyway, than the Dom:

https://www.tripadvisor.com/Attraction_Review-g187393-d192295-Reviews-St_Stephan_s_Church_Stephanskirche-Mainz_Rhineland_Palatinate.html

You'd probably want to stop in at the Augustinerkirche too, in the heart of Mainz' attractive old town zone on Augustinerstrasse:

https://www.romantic-germany.info/infrastructure/infrastruktur/Augustinerkirche_Mainz/deskline-details.html

Go to Google maps and look for Römer (the city hall) in Frankfurt, then click on the photos. It's exactly what I imagined a typical German town would look like (and most don't). Frankfurt is one of the most underrated tourist destinations in Europe.

The Stephanskirche in Mainz has superb Chagall windows.

With the help of your wonderful tips, I’ve come up with this itinerary. Does it seem reasonable?

Friday: Train from Amsterdam to Boppard, stopping off in Cologne to see as many of the following churches as I can: Cologne Cathedral, Great St. Martin Church, St. Andrew’s, Basilica of the Holy Apostles, St. Gereon, The Basilica of St. Ursula. I sure hope I can leave my luggage at the train station. Does anyone know if the Cologne train station has a luggage check or lockers?

Saturday: Train from Boppard to Bingen and then ride a boat back to Boppard, stopping off in Bacharach, Oberwesel and St. Goar.

Sunday: Explore Boppard in the morning and then train to Frankfurt.

Monday: Städel Museum, Frankfurt Cathedral, Deutschordenskirche, Karmeliterkloster, St. Leonhard

Tuesday: Day-trip to Frankfurt-Höchst and Mainz to see Saint Justin's, Mainz Cathedral, Augustinerkirche, and Stephanskirche

Good plan, just be aware that St. Leonhard's will not be open on Monday and St. Justinus in Höchst will only be open from 14:00-17:00. If you are in Höchst early though, their market is on Tuesdays from 7-13:00 including their market Hall, which is quite nice with lots of local farmers. Or you could see Mainz in the morning and visit Höchst in the afternoon? The garden and the tower behind the Justinus church are worth a visit.
Maybe pop into the Alte Nikolai since you will be there in the Römer area anyway.