Klausen/Chiusa (Italy) to Amsterdam.

You should take the middle of the Rhein-bike-trail starting south of Mainz up to Koblenz or even Remagen. Extremely scenic route (although often very close to a highway, the valley is narrow and very, very busy) - take stops at the countless castles along that section, see the remains of Remagen Bridge.
Support the Ahrtal (west of Remagen) after their devastating flooding last summer
Visit some of the largest open-air-coalmines between Aachen and Cologne