You can visit Burg Rhinefels from the KD steamship line. It is (barely) walkable, or more easily a very short cab ride from the dock.
It began to rain on and off as we followed the Mosel, then the Rhine back to St. Goar and Rhinefels castle. With limited time we took a quick tour of Rhinefels castle, which is in ruins. It was interesting walking through the ruin and trying to imagine what it would have been like (coming direct from Burg Eltz helped with that). Rhinefels is huge and impressive. It is much more spread out than Burg Eltz (which grew up). It was raining when we were there, we would have stayed a LONG time if it had been sunny.
The pictures say it much better than my words do.