After putting long list of different German monuments, which are not so interesting to my mind, finally in 2002 they remembered Rhine valley. It had to be one of the first to come from Germany!
If you drive a car (I had to), then it would be a problem. Road is winding, and you should be very careful all the time - but the views around are incredible! Nature has made deep and magnificent valley and people have developed here excellent landscape dotted with castles and lovely towns and villages. Here you can start to believe that there is harmony between man and nature - at least visually.
True - it is worth to drive also along the right coast of Rhine - less traffic and magnificent views. On some places try also the upper road along the right coast.