Bischofstein
Bischofstein and the town of Burgen are situated south along the Mosel River as one heads towards Treis and Cochem.  While not phenomenal-- I'm not even sure if you can visit the castle, it still provided a beautiful example of a small German town, with virtually no foreign tourists.  The campground had only a handful of people there, and I saw no evidence of another American in the area!  However, at selected times of the year the area is inundated by students from the Fichte-Gymnasium in Krefeld (the current owners of the castle) as they go on retreats to the castle.
Peacefully idyllic, as swans silently glide about on the gently flowing river below the castle.  As the castle is now a retreat center, most is not open to visitors.  However, the tower and other ruins are open to those who wish to make the trek.

As for Burgen (below), the town is replete with narrow streets and quiet charm.  This is an out-of-the-way location, and has none of the characteristics of tourist traps.
On the hills above Burgen, one can be treated to the beauty of the Mosel and its uniquely steep slopes (steeper than along the Rhein).  These vineyards are difficult to work, and those doing the labor are provided with small rail-mounted carts to move the grapes downhill to a collection point.  For a view of one of these carts see the section on Cochem.
Originally built in the 1200s, it was once the property of a church leader in Trier, until he handed it over to the Church in that city in 1273.  It at one point provided protection for an ancient Roman trade route along the river, collecting tolls as a means to sustain its operations.  Sieged numerous times in the Middle Ages, it finally succumbed to French forces in 1689, remaining as a ruin until the 1930s, when a private businessman had it rebuilt for a summer home.  The castle was used as a rehabilitation center for German soldiers during World War II, and in 1954 it was sold to become a student retreat center.  While the dormatory portion is fairly new, the tower and other ruins date back to the middle ages.