We are going to Amsterdam, then driving through Belgium, Luxumbourg and into Germany, where would be a good place to have a day trip up the Rhine?
The most scenic route is through the Rhine (Loreley) Gorge and a popular ferry route is from St Goarshausen to Rüdesheim. St Goarshausen is about 20 miles south of Koblenz on the right bank (east side of the Rhine.
My Husband and I are doing a very similar trip; go on line to www.k-d.com and check out the cruise company. They also have a 1913 steamer. You can get off at any stop and then return the other way. There are a number of castles to explore.
You did not say where your day trip on the Rhine river will begin. I recommend go to St. Goar in Germany. Be at a hotel there for two nights. There I liked Hotel Hauser. I would ride in a train from St. Goar to Bingen. (Leave St. Goar at 9:00 a.m. or earlier). Ride on a ferry boat across the Rhine River from Bingen to Rudesheim. Ride in a chair lift up hill to the Niederwald monument. Up there you have a great view of the Rhine River valley. And you can see a nearby castle ruin at that side of the river. Return to Bingen. Ride on a big boat on the Rhine River from Bingen to St. Goar. If you start early in the morning, ride in a train from St. Goar to Bacharach, walk through Bacharach for 45 minutes. I guess Bacharach is the most charming small town on the Rhine river. And ride in train from Bacharach to Bingen. That section of the Rhine River (Bingen to St. Goar) is the most scenic. You will see castles at both sides of the river. It is one of the world's great boat rides.