Researching best itinerary options for 2-4 days. My husband and I (60’s) are planning 25 days via trains in Europe in mid-April. Since other parts of the trip will be in larger cities (London, Amsterdam, Prague, Vienna, Budapest), we would enjoy smaller but interesting towns and beautiful scenery. If possible, integrating one or more easy bike rides and short river cruises would be great. Looking for suggestions for best itinerary, with convenient transportation. We are not history or museum buffs. Most of our lodging will be booked with hotel loyalty points so actually staying in larger cities to use as a base works best but not opposed to other choices on this part of the trip if that seems better.
I've just responded to your question on the general-europe forum. Here's the link:
https://community.ricksteves.com/travel-forum/general-europe/need-a-lot-of-help-25-days-in-europe
On your itinerary, between Amsterdam and Prague is Berlin and Dresden. Needless to say Berlin is a great large city that is of great importance. But Dresden was perhaps the most beautiful city in Europe before Allied bombings the end of WWII.
Note that the Netherlands is well-supplied with attractive smaller cities, like Amersfoort, Deventer, and Nijmegen. Eastern Belgium has some lovely old medieval town centers, but I don't think the detour is justified because there are so many others.
Munster, Germany is not small, but very attractive and lots to see. I consider Cologne a superb, multiday visit, and it's a likely train hub for your trip. Not an attractive part of the Rhine, however, so boat rides not essential right there-you can find many posts here on the Middle Rhine scenery, easily accessible on your route. Eisenach, Erfurt, and Weimar are extremely attractive places.
I did not look into it, but it seems like it should be possible to take a boat most of the way from Dresden to Czech Republic. Very scenic ride in Germany, optional vineyard or porcelain stops.