The Mosel River is one of the least appreciated areas of Germany, and this can be a good thing!  For one, it is not quite the tourist trap as the Rhein, and second it offers beautiful scenery and awesome castles, though not as many as the Rhein.  The vineyards along the river are more dramatic than the Rhein as well.  Indeed, the Mosel is the hidden jewel of western Germany, and a must visit.