germany itineary

Take the train from Amsterdam down to Cologne for a tourist stop. Pick up a rental car there and drive up the Rhine River stopping at Koblenz and Bacharach along the way. (See http://www.bensbauernhof.com ) Your road trip will be just about perfect for 8 days time.
My best suggestion is to not cut your time short in Munich as there are so many things of interest to see there.

As the previous poster noted, you would be better off renting the car somewhere in Germany to avoid a foreign -off fee. Here's a suggested route starting from the Köln/Bonn area. Drive south along the Rhine. North of Koblenz, it doesn't matter which side, but I would recommend driving on the east bank south of Koblenz, because you'll be able to see the more numerous castles on the other side of the river better, and because the best castle to tour (Marksburg) is on this side. Continue along the Rhine until you hit the Wiesbaden area. Link up with the A5 Autobahn (via A3 and A67), and head south towards Heidelberg. Between Darmstadt and Heidelberg, A5 runs parallel to the Bergstraße (mountain road) district, which features several attractive towns, vineyards, the western edge of the Odenwald mountains and several castles. The view from the Autobahn is quite nice, but if you want to get closer for a slower drive, you can drive on the Bergstraße itself (B3). Towns worth stopping for a quick visit include Zwingenberg, Bennsheim, Heppenheim and Weinheim. This entire region is featured in the current issue of German Life magazine. (cont.)

From Heidelberg, continue on B37 up the Neckar river valley towards Heilbron. This is similar to the more famous Middle Rhine Valley, but I think several of the towns here are nicer, including Dilsberg, Neckarsteinach, Hirschhorn, Eberbach, Mosbach, and above all, Bad Wimpfen. At Heilbron, head east on A6. Consider stopping at Schwäbisch Hall and/or Crailsheim. If you still want to see Rothenburg, head north on A7 after it links up with A6. However, if you stopped in more than one or two of the towns I already mentioned, you may find Rothenburg a little redundant, unless you want to see crowds of bus tours. For most of it's length, the Romantic Road is pretty indistinguishable from any other rural road in that part of Germany, except for the higher tourist traffic. The scenery of the drive itself is nothing special, so I wouldn't go out of my way to drive on it. Dinkelsbühl and Nördlingen are the two stand-out towns south of the A6-A7 junction. I'm less familiar with the route from there to Munich, so perhaps someone else can make suggestions.