We traveled from Mainz up to St Goar by bus, then cruised to Boppard before taking the bus to Koblenz. After lunch we crossed the Rhine and returned to Mainz via the narrow streets of Rüdesheim and the Germania monument. There were dozens of Castles all around the Rhine.
Read more on the (Castles of the Rhine link).

Mainz Dom
Traveling the Rhine
We had a short 1 hour cruise from St Goar to Boppard. The rest of the time from Mainz to Koblenz and back, was by Bus.

Sooneck Castle

Fürstenberg Castle
Stahleck Castle
Pfalzgrafenstein Toll Station
Gutenfels Castle
Schönburg Castle
The Lorelei
Katz (Cat) and Maus (Mouse) Castles

Rheinfels Fortress

Sterrenberg Castle

All-in-one bar church. The only way to get to the bar is through the church!

Former Prince Elector's Castle in Boppard
Marksburg Castle
William I monument at German Corner (Deutsches Eck) The point where the Mozel (left) joins the Rhine

Borrowed image from interweb
Some of these look like they are from Count Dracula's neighbourhood
The Niederwalddenkmal (Germania Monument)
View of the Rhine looking towards France from the Germania Monument