If you like World War 2 history there is an interesting museum on the west bank of the Rhine at Remagen, literally in what's left of the famous bridge. The castle at Burg Eltz is supposed to be good, and you could also go up to Cologne for the cathedral.
I have a route planned around the area from a trip my brother and I did a few years ago. Part of it was following the road along the Mosel, from Koblenz to Cochem, but it really was so dull, that we picked out a new route using a Michelin map whilst stopped for lunch at an Imbiss. Picked out a couple of waypoints for the GPS and we had a great afternoon's ride back up into the hills :-0)
Here it is anyway, in case it's any use to you.