Rather than just getting in the car and setting out, and stopping based on your impressions from the driver's seat of your car, I'd suggest a more deliberate approach. Do some thoughtful research based on your own interests and on the experiences of millions of folks who have already explored this region.
And don't feel restricted to the Rhine and Mosel River valleys, even though most sights tend to be there. It's just possible that you'd be curious about the amazing collection of buildings in Bad Sobernheim known as the Rheinland-Pfälzisches Freilichtmuseum, or Germany's finest racetrack (Nurbürgring) or the Hinzert Concentration Camp memorial site, or the intact German WW II B-Werk bunker in Besseringen, or gem-town Idar-Oberstein, or nicely-preserved Limburg on the Lahn River.
This guide breaks the area down by region and though not exhaustive is pretty complete and would be a good start.