Scenic day drive Cologne to Frankfurt suggestions?

Just follow the Rhine, at least starting to the south of Bonn. That should be the better part of a day.

I've never been, but if you have a car, Burg Eltz - Rick's favorite European castle - appears to be a doable detour. You could also create your path to pass through Bacharach and Sankt Goar and that part of the Rhine corridor.

Agree - follow the Rhine from Bonn south. I like Burg Eltz but I wouldn't make that detour as you can take a tour instead at Marksburg Castle in the Rhine town of Braubach, just south of Koblenz. I would arrange to arrive in Braubach at around 11:00 or so to see the town and scout out a place for lunch on the Marktplatz (market square) in town below the castle. Then be at Marksburg for the scheduled 1 pm tour (in English) - there's a lot to see there. Marksburg provides a look at medieval life inside a never-destroyed medieval castle which has stood for nearly a millennium. Rick includes Marksburg in his Rhine suggestions - see article - but please ignore Rick's incorrect assertion that English tours there are not possible. They in fact happen every day. Even if you cannot catch the scheduled 1 pm or 4 pm tour, there is often a tour group getting an English-language tour which you may be able to join - just like the Marksburg website makes clear.

After Braubach and Marksburg you can drive south to the Filsen ferry crossing (just south of town.) That takes you and your car to Boppard - a nice riverfront town - and then continue south to St. Goar, Oberwesel, and Bacharach, all of which, I agree, are worth seeing. You just have to study up a bit to see what interests you the most as you likely won't have time for everything. I took my wife on the chairlift ride in Boppard recently - she was awed by the view from up top at Gedeonseck terrace. But you may be more interested in castle ruins, churches, or a walk around old-town fortifications... so much to do here if you had more time.

Agree with Russ-the tour at Marksburg was great, and the chairlift in Boppard up to the panoramic lookout was a highlight of our 3 days in the Rhine valley!