Arriving FRA Saturday 14:35, recs for best Rhine base for about 7 days by public transport pls?

If you stay in Boppard, you will receive a VRM pass to ride all the regional trains and buses in the district for free during your stay. Located on the Rhine with a KD dock. The VRM district extends north of Koblenz, up the Mosel River well past Cochem,

The RE2 goes direct to Boppard from FRA Regionalbahnhof at 24 past the odd hours, so if you luck out on arrival time and luggage collection and passport control, you might make that on at 15:24. It takes 1 1/4 hours. Missing that, use the S-Bahn to Mainz on the half hour (15:30, 16:00, 16:30) and connect to Boppard from there, possibly with an additional connection. That takes 1 3/4 hours. Cost will be 59 EUR for 2 people using a QdL ticket.

There is much that your RS guidebook does not tell you.

Boppard is a very good choice. The VRM Pass goes to all guests. Covers trains AND buses for free. Map of VRM travel area and details:

https://www.vrminfo.de/fileadmin/data_vrminfo/PDF/Gaesteticket/Infoflyer_Gaesteticket_englisch_2022.pdf

Here is an orientation map for the Rhine Gorge with train routes, towns, cruise boat docks, etc.:

http://www.loreley-info.com/eng/rhein-rhine/walking-hiking.php

K-D runs a reduced schedule on Mondays for the most scenic cruise segment. Most boats travel Bingen > St Goar. Only ONE K-D boat travels Bingen > Boppard directly (without a change in St Goar.) So a good way to arrange your cruise day is to take the train to St Goar, Oberwesel, and/or Bacharach - especially these latter two - for scenery, walks and old-town photography. Then continue on the train to Bingen Rhein Stadt station and catch the 16:30 northbound boat to Boppard; this is one of the boats that does NOT run on Mondays, so do it some other day.

On this day, your VRM pass will cover southbound train travel from Boppard to St Goar and Oberwesel, but not beyond. You should purchase normal DB tickets for Oberwesel > Bingen Rhein Stadt to supplement your VRM pass in order to travel at will on the railway south of Oberwesel. A normal ticket allows you to hop off in Bacharach and back on later if you wish and costs €8.70 ea. Buy at the station in Boppard or on the DB app prior to boarding your southbound train that day. Note that your train ticket will provide you a 20% cruise discount if you present it at the KD ticket kiosk in Bingen... so don't pre-purchase those cruise tickets, just arrive a bit early (maybe 30 minutes?) so that you can make the short walk to the kiosk and stand in line for a bit as needed.

DB site:
https://int.bahn.de/en/

FRA airport > Boppard Hbf: Leave from the FRA Regionalbahnhof station (Frankfurt(M) Flughafen Regionalbf) on a regional train (S, RB, or RE.) There is a direct train to Boppard at 15:24 and 17:24, but you will probably be leaving in between those hours, in which case you can just catch the first possible regional train to Mainz Hbf, where you transfer to a Boppard bound train as the schedules dictate.

The €59 Day ticket for Germany (QdL ticket) covers 2 persons and will get you to Boppard on ANY regional train or combination of Regional trains that day. You can buy it once you get there at a station ticket machine or using the DB app.

https://int.bahn.de/en/offers/regional/day-ticket-germany

Boppard worked well for us when we did that.

I provided ideas for your arrival day and "cruise day" previously. Options for fleshing out the "cruise day" below:

• Town wall walk in Oberwesel:
  https://www.tourenplaner-rheinland-pfalz.de/en/tour/urban-walk/oberwesel-city-wall-circuit/56218092/
  https://www.tripadvisor.com/Attraction_Review-g198501-d2189241-Reviews-City_Fortifications-Oberwesel_Rhineland_Palatinate.html

• See R. Steves' suggested walk in Bacharach... also... map below, town wall walk drawn in yellow ("Stadtmauerrundweg" = town wall loop.)
  https://www.informagiovani-italia.com/mappa_bacharach_am_rhein2.jpg

On another day...

The 3-hr. walk below targets one segment of the "Rheinburgenweg" (Rhine Castle Trail) and is one of the finest walks I've taken in Germany. You could do this on a separate day from the town wall walks. I did it in reverse of the suggested direction... train to St Goar, maybe visit Rheinfels Castle ruins, walk south to Oberwesel (reverse direction of this route), train back to Boppard (covered by the VRM day pass or the D-Ticket.) This route excludes Rheinfels Castle (steep ascent with staircases from the St Goar train station to the trail atop the cliffs):
https://www.tourenplaner-rheinland-pfalz.de/en/tour/long-distance-hiking/rheinburgenweg-stage-4-oberwesel-st.-goar-south-north-/1526949/

This route includes Rheinfels:
https://www.outdooractive.com/en/route/hiking-trail/romantischer-rhein/panorama-trail-st.-goar/58218012

If you begin this second route in St Goar, you can avoid the steep ascent of the suggested trail by walking along Schloßberg (the more gradual road up to the castle) or catching a bus from town (3 different buses stop there.)

On another day...

Use the train (VRM pass or D-Ticket covers everything) to visit the east bank of the Rhine. Rüdesheim at the southern end and Braubach in the north near Koblenz are both worthwhile destinations. It's easy to find information on R'heim... Not so much on Braubach, so see links below. Some photography of Braubach on 3 pages at the following link:

https://www.stadtbild-deutschland.org/forum/index.php?thread/7322-braubach-am-rhein-und-marksburg-galerie/

A pretty good take on visiting Braubach: https://delveintoeurope.com/braubach-germany/

Marksburg Castle in Braubach is the one premier Rhein castle for touring. English tours normally are at 1 and 4 pm. It's a fairly steep walk from town up to the castle entrance, but not too long (25 minutes?)
https://www.tripadvisor.com/Attraction_Review-g642094-d268085-Reviews-Marksburg-Braubach_Rhineland_Palatinate.html
https://www.marksburg.de/en/circuit/#/

Other places for day trips or half-day trips: (RS book may help in spots.)

REMAGEN: a nice town for easy walking. Covered by VRM Ticket or D-Ticket... The Remagen waterfront promenade is lined with lunch spots. One of the St. James Way pilgrimage routes runs through Remagen; you can easily walk from town to the Apollinariskirche (church there, see link below) and to the statue of St Francis just uphill behind it for a nice view of the Rhine. Walking tour route and sights:
https://www.yumpu.com/de/image/facebook/4301709.jpg
https://www.romantischer-rhein.de/en/a-apollinariskirche

• Bridge at Remagen "Peace Museum," (WW II of course, but not gruesome, interesting tale about the struggle to cross the Rhine and the local community.) https://www.tripadvisor.com/Attraction_Review-g642105-d2165317-Reviews-Peace_Museum_Bridge_at_Remagen-Remagen_Rhineland_Palatinate.html

• Arp Museum (art) north of Remagen at Rolandseck station: https://www.tripadvisor.com/Attraction_Review-g642105-d729455-Reviews-Arp_Museum-Remagen_Rhineland_Palatinate.html

COLOGNE: Germany's most visited landmark (cathedral) and much more. Direct trains from Boppard. Fully covered by the D-Ticket.
COCHEM + cruise to Beilstein (Mosel River.) Use either the VRM ticket or the D-Ticket.
BURG ELTZ (Castle near Cochem)

My favorite town on the Rhein is Eltville. Charming promenade along the river, lots of good restaurants and wine, beautiful old church, a dry moat filled with roses, and a short bus ride away from Eberbach Monastery. This is one of the top places on the Rhein to visit, in my opinion. The basilica here is magnificent. The filmed the Name of the Rose at this monastery. Stop by Kiedrich too and perhaps see St. Valentines church.

Not overly fond of Rüdesheim, but it is fun to take the gondola ride up to the Niederwald Denkmal. If you can go at sunset, the views are stunning. For wine tasting, head to Rhein Wein Welt, which has a unique way of offering it, in rooms that were once the Asbach factory. I do like Siegfrieds Musical Cabinet as this also offers something special you won't find anyplace else.

If you are at St. Goar, do visit the church. It is beautiful and one of the nicest along the Rhein.

Visiting either of the Werner Kapelle in Bacharach or Ober-Wesel? Take some time to learn why these chapels are here and who Werner was. Rick does skip over this part in his guide book.

Thank you so very much for all of the suggestions, this is so helpful! Accommodation options are pretty limited at this point but we've found an apartment in Bacharach for six nights. Digging out maps now to look at all of your great advice.

Eltville makes for a good visit if you are based in the city of Frankfurt (direct train, 1 hr.) but from Boppard it'd be 2 hours each way on the train. Rüdesheim isn't for everyone - mostly because everyone goes there and it gets pretty crowded/touristy. It is a bit more accessible in about 1.5 - 1.6 hours one way and combines well with Braubach/Marksburg. The main advantage I see for R'heim is that it is still within the scenic Rhine Gorge - and you can enjoy the scenery using the cable car, the hiking options, and/or the chairlift ride to Assmanshausen

https://www.ruedesheim.de/en/cable-car
https://www.komoot.com/collection/1312300/between-rhine-and-wine-hiking-around-ruedesheim-in-germany

If R'heim isn't what you want, there's a cable car ride much closer to Braubach in Koblenz, which takes you across the Rhine to Ehrenbreitstein Fortress. Also a nice thing to do and the views are quite nice.

https://www.seilbahn-koblenz.de/homepage.html

Not far from the cable car station in Koblenz you can find wine venues like this one and others (on Tripadvisor.)
https://www.tripadvisor.com/Restaurant_Review-g187391-d17730057-Reviews-Drei_Hefen-Koblenz_Rhineland_Palatinate.html

Bacharach will add substantial travel time to the Braubach, Cochem Burg Eltz, and Cologne journeys outlined. It's just not an efficient base town for these trips. Please ignore my suggestions on the VRM Guest ticket for free train travel - It's not available in Bacharach.