Koblenz is a city of two rivers, and most of its attractions are arranged along them. The headland where the Moselle meets the Rhine — the Deutsches Eck — is the obvious starting point; from there the cable car lifts you over the Rhine to the enormous Prussian fortress of Ehrenbreitstein for the best view in the Middle Rhine.
The rebuilt Altstadt between the two rivers is small enough to walk in a morning, with a cluster of churches and squares that survived or were reconstructed after 1945. The real reward, though, is using Koblenz as a launching pad for the Rhine Gorge and the Mosel valley — both UNESCO World Heritage sites are on the doorstep.