Aachen is a superb destination, and easy to get to. Two other options are Dusseldorf, which has its own local beer, completely different from Kolsch, and Monschau, which is touristy, but an outstanding, very large and pure half-timbered town. May not have a train station. Some people, especially those who've seen the German movie "The Princess and the Warrior" want to see the Schwebebahn in Wuppertal, just beyond Dusseldorf.
Note that Cologne has more than two full days of attractions itself. It is completely false to say that there is nothing to do in Cologne except the highest-turnstile count place in Germany, the Dom. And many who visit the Cathedral skip the superb Treasury because it has an entry charge.
https://community.ricksteves.com/travel-forum/germany/can-i-see-enough-of-cologne
Many people would mention the scenic Middle Rhine boat ride from Bingen to Boppard, which is do-able, but easier from Frankfurt, for example. The train is always used to get to and from this boat ride, not the "boat all the way". Maybe that's on the Rick tour???