YOu can drive to Koblenz along Bundesstraße 9 (left side of Rhine) or Bundesstraße 42 (right side of Rhine). The A 66 ends on the Bundesstraße 42 near Eltville while A 60 at exit 13 (Bingen) leads to the Bundesstraße 9.

I have driven both Bundestraßen many times (it is fun in a convertible in summer). Beware, there could be many cars on this streets, especially if the weather is fine, but is couldbe also awful if you have bad weather. The Rhine Valley ends in Koblenz. There are a lot of parking spaces near the Rhine, just follow the signs for "Deutsches Eck".

If you start in the morning, you should calculate two hour by car from Frankfurt to Koblenz via Bundesstraße.

I have driven along Neckar twice, but both times we had heay rains and I had to look about the road and therefore cannot remember anything about landscape.

In Frankfurt you will finde some German (or "Hessische") Restaurants in Sachsenhausen (e.g. www.dauth-schneider.de; www.zumgemaltenhaus.de).