The center portion Main Valley, as it cuts through the Odenwald and Rhön Mountains, is an underrated region to explore. I tried a short hike in the region, but I probably would't recommend it to others.
Life can be unfair, and I think Liege (or Lüttich or Luik depending on who you ask) is a good example of that. The city is rather nice. Its not as pretty as Ghent, and its certainly way nicer than Cologne. I would put it on par with Antwerp and Brussels. So why then does nobody go there?
Overall though, I can highly recommend the hike to anyone who might be interested. The Ehrenburg castle is fairly unique in Germany. The ruin is fairly well preserved, the French having opted to burn rather than detonate the castle.
This castle looks really cool, but its also really inaccessible. Most of the castles in the region are either fragmentary ruins or 19th century interpretations of a castle. This combination appears to have worked out well enough and might be worth a visit.
I recently acquired several new books, one of them is an architectural guide to the castles of the Taunus. As is typical with Germans, the title is boring, uninteresting and describes precisely what is inside: Castles of the Taunus.