If you have a car, and you're thinking you need a really good castle not too far from Koblenz, take a short ride to Schloss Bürresheim, a 13th century castle, never destroyed, lived in until 1921, and still furnished with the original family's belongings. And you'll see 3 times as many rooms as you would at Burg Eltz, the castle is arguably prettier, and Americans don't even know it exists. It was used as the set for the movie Indiana Jones and the Last Crusade. It's easy to walk around, much easier than Burg Eltz, and it's cheap. A tour only costs 6 euro.
Even better, you'll only be 6 miles from Mayen, which has another good castle, great city walls and gate towers, and an excellent marktplatz in the center of the old town. And if you drive another 10 minutes you can go cave diving under the lava dome, and then visit Volcano Brewing, a brauhaus that's been around for hundreds of years.