Day trip fron Frankfurt Airport to Rhine River - car recommendations?

I cannot answer your question about private drivers.

The Rhine castles of interest which you can tour are Marksburg (in Braubach, an intact medieval castle) and Rheinfels (in St. Goar, a set of castle ruins with museum.)

Map showing towns, train lines, ferry crossings, cruise boat stops:

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

I wish I could help you with a car service but when I go to Frankfurt several times a year (and along the Rhine) I have to do it in my own car for medical reasons, so don't know any drivers.

I would certainly suggest giving the train another chance because it is likely to be a more productive day than by car. The traffic west of the airport and west of Frankfurt can be awful (personal experience, often) and there is frequently significant disruption due to bridge and road works. As you go back and forth on the B9 you will be low down at car level - on the train you can sit upstairs on many trains and have a great view. The train stations are convenient and the trains frequent.

If your airport hotel is in Gateway Gardens there is an S-Bahn train station right there which has frequent (at least 4 times an hour) trains to Wiesbaden which on the way, or you can go one stop back into the airport where you can get both slow and fast trains to the areas mentioned by Russ.

I'm sorry I'm not more help with your question.

catmilha, it was my experience that the KD boat had a better view than a train or car, and high comfort level for seeing a huge number of exterior-only castles in one day. It also was a nice experience, with sausages and beer for lunch on the boat. We left our car at our hotel, and found the local train to be quite fast. I admit that we had no interest in castle interiors, but there are only a very few you can enter. We did include Burg Eltz and the Wartburg on the same trip, which was enough castles for us.

I personally would stay in Frankfurt, not at the airport. But you have not sketched out your actual itinerary for us. Have you been to Europe before? You might want to add the Lorelei to your Rhine list.

Mainz is often recommended here as a base, and it can be reached on the no-reservation S-Bahn directly from the airport. I wonder if it might actually be easier, from a full-service hotel in Mainz, to book a car service to the Bingen KD dock. You would have to get back to Mainz from, say Boppard, probably on a regular train.

I personally would stay in Frankfurt

We are flying into Frankfurt in May and staying a couple of nights,
hoping to see some of the Rhine River castles during the day we are
there.

With the Rhine as your main objective, and with your hotel in the Frankfurt area, your struggle here is that you are loading too much into your single sightseeing day. On day 2 you are attempting to line up... 1) your transportation from the Frankfurt area to the Rhine Gorge, 2) the castle visits, 3) the river cruise, and 4) the transportation back to your Frankfurt-area hotel. It's a bit much.

There are tours by Viator and others that will take people who strand themselves in Frankfurt to the Rhine Gorge and back in a single day, but I like the idea of being more independent and seeing MORE by yourselves than a tour can offer.

Tim has suggested an S-Bahn train from FRA to Mainz, (on Day 1, presumably) then on Day 2 a personal shuttle to the dock in Bingen, then returning back to Mainz on another train. The trains are sometimes late these days, but they really aren't all that tricky, and I think his suggestion is doable. Still, while it be an improvement over your plan... I'm thinking you could be more efficient than this... if you can do a train or two, why not skip the private shuttle? And instead of Mainz, why not stay where the cruise begins?

Day 1: There are a few direct morning trains from the FRA airport train station to Bingen... at 7:10, 9:24 and 11:24. Then in the afternoon, at 13:24 and 15:24. The trip to this Rhine Gorge gateway town takes only 45 minutes. Direct trains will not only shorten your ride but eliminate changes of train which can be missed.

I suggest booking your 2 nights at the nH-Bingen hotel, which is right on the attractive waterfront promenade where the cruise boats dock and where the Nahe River runs into the Rhine.

https://dynamic-media-cdn.tripadvisor.com/media/photo-o/06/1a/df/01/nh-bingen.jpg?w=900&h=-1&s=1

From Bingen's main station, the walk to the nH-Bingen is flat and doable. Or you can catch a taxi to cover the short distance instead. The hotel has restaurants nearby and a fine view of the river and Ehrenfels castle ruins across the water.

If you get to Bingen early on Day 1, you might fit in a cruise that afternoon. There should be afternoon K-D cruises from Bingen to St Goar at 14:30 and 16:30 (most days.) This is the most scenic part. The 14:30 cruise would permit you to return on the same boat to Bingen. (The 16:30 reaches St Goar at 17:55, which is too late for cruising back. But there are hourly trains back to Bingen you can use.)

On Day 2, you could experience a very full day of the Rhine that includes both tourable castles...

1) ...take a direct morning train (20 minutes, 8:39 - 9:00?) from Bingen back to St Goar for a tour of Rheinfels. Have lunch.

2) ...ferry across the Rhine from St Goar to St Goarshausen (5 min.) Then take the Direct train to Braubach (20min.)

3) See town. Take 4 pm Marksburg Castle tour (in English.)

4) More cruising? Take 18:55 K-D cruise to Boppard (19:45.) Then take the direct train back to Bingen (25 min.)

If a cruise cannot be fit into Day 1, then perhaps a more rushed Day 2 with a cruise and both castles could be possible. All times provided are from the current schedules, so they are subject to change for your May '25 trip, of course.

Thank you everyone for the great advice. This gives me a lot of information to use in planning! Appreciate the help.