Rhine River

Best daytrip includes a visit to Marksburg: Get off in Braubach for Marksburg. Start in Bingen (around 2.5 hours) or Bacharach (around 2 hours) for the best scenery on the cruise. Cruising south to north makes for a somewhat quicker trip (downstream) so you can spend a bit more time exploring this or that village (Bacharach, Boppard, and St. Goar are all fun.)

"what portion of river best for one day trip?" Are you asking what portion would be best to tour for one day while staying somewhere in the Rhine gorge, or as a daytrip from somewhere else?

I've already spent four+ days in that area, so I am not likely to do it again soon. However, if I were to, I'd start in St. Goar with Rheinfels castle, then take the train to Bacharach and explore it, then take the boat down the Rhein to St. Goarshausen and take the train to Braubach and the Marksburg. I took the boat from St. Goar to Bacharach, and I consider it a major mistake. I can read a map, I knew Bacharach was up the river from St. Goar, but I had no idea the strength of the current and the extra time doing it in that direction. I was really bored by the time I got to Bacharach. As far as I am concerned, the only really worthwhile part of the river is between St. Goar and Bacharach. Most of the sights you can see a lot more effectively from the train, but seeing the Pfalzgrafenstein, the mid-river toll castle from the river, is so much more impressive.

I'm with Lee, but I'd stretch it a bit at both ends to include Boppard and Bingen, but that's too much to do by boat in either direction.

Two comments: 1. There are two train stations in Bingen, the Hauptbahnhof and the Stadt (City) Bahnhof. The Stadt Bahnhof is closer to Mainz and also close to the boat docks. The Hauptbahnhof is on the other side of the Nahe river, a considerable walk to the boat docks. There are buses from the Hauptbahnhof, but you have to know where to catch them. 2. Boppard is the largest town between Koblenz and Bingen and has some sites of interest, including the ruins of 4th century Roman fortifications. However, it's on the opposite side of the Rhein from Braubach and the Marksburg. There is a tiny car ferry across the Rhein from Boppard, but it lets you off at a deserted section of the right bank highway, about 1 km from the Bahnhof in Filsen. Second option, take the train to Koblenz and cross the river there to Braubach by train or bus.