Oh, wow. You are speaking my language. We have been up and down this corridor A LOT the past couple years. Tell me that's a typo and you have "an extra 2-3 DAYS" not hours!! OK, not knowing your interests, I'm just going to put everything out there and you'll probably have time for one. But you'll be spoiled for choices.
Just north of Cochem is a very cute, half-timbered town called Linz am Rhein. Walk around and make your way to the Buttermarkt to soak up the atmosphere. Then head to La Cucina Restaurant Linz for a fantastic lunch in a great, old building overlooking the river.
In Bonn I 100% recommend the House of the History of the Federal Republic of Germany. This is an outstanding museum -- one of the best in Europe. It tells the history of West Germany, from the end of WWII to the fall of the Berlin Wall. Then, across the River on the "Beuel" side in a gem of early medieval architecture. St. Maria & St. Clemens is a Doppelkirche or double church. Both stories are filled with Romanesque 1100s frescoes that were only rediscovered in the 1800s. Both stories have undergone modern restoration and the results are incredible.
As for Cologne itself, I'm going to give you a bunch of church destinations. You may or may not want to do a bunch of church destinations, but each is outstanding in their own right:
- Cathedral, esp. the Reliquary of the Three Magi. Astonishing when you consider it was crafted in the early 1200s
- Kirche St. Ursula. Find the attendant in the side office, pay the 2 Euros each, and head into the reliquary decorated with thousands of bones. Mind-blowing!
- St. Maria im Kapitol. Wooden doors from the 1000s, considered some of the earliest and best wooden reliefs of medieval Europe
- Church of St. Maria in Lyskirchen. Another 1200s-era artifact, this time for the outstanding painted ceilings.
- St. Gereon. Quite peaceful and beautiful, and pretty amazing when you realize the dome was the largest built in the world between its completion in 1227 and the 1400s when the Duomo in Florence was completed.
Okay, enough churches! Final recommendation: There are lots of traditional Kölsch halls, but my two favorites are: Schreckenskammer (Ursulagartenstraße 11-15) and FRÜH "Em Golde Kappes" (Neusser Str. 295) -- requires a trip up to one of the quieter neighborhoods. [Maybe you don't want a Kolsch hall if you are driving, but....]
Finally, I would strongly advocate for a westerly detour to Aachen. Aachen Cathedral and the Treasury absolutely one of the most significant sites in Europe.
Between all of those, you can hopefully find the one thing to fill the time!
