When my friend remydoodle visited this summer, we took two cruises on the Rhine. One was a one-hour trip and the other one was a seven hour trip. You can book all kinds of cruises with various companies, and if you ever visit Koblenz, I'd highly recommend to at least take one trip.
There are a lot of castles--some are in private hands, some are open to the public--and ruins scattered along the Rhine. Here are some pictures:
These two castles below are called "The Hostile Brothers". In case you're interested in their story, please visit this website.