Recommendations on 1 1/2 days in/around Koblenz before GAS tour

I am sure on the first day you will be a little wiped out getting to Koblenz. I would suggest just exploring Koblenz that day. The second day I would recommend taking the train to St, Goar and a cruise on the Rhine from St. Goar to Bacharach. After the cruise you can walk the town of Bacharach and train back to Koblenz. You will need to wait for the summer cruise schedule to plan. Enjoy your trip.

Coming from FRA? I would look for one of the direct regional express trains from the FRA regionalbahnhof to Koblenz Hbf (9:24, 11:24, 14:24) which cover the distance in 1.5 hours. Sitting on the right side of the train will provide the best Rhine Gorge scenery.

Your tour includes some sightseeing in Koblenz already; on Day 1, I'd suggest taking the train from Koblenz Hbf to the Rhine town of Braubach (only 13 minutes away.) It's a small, picturesque little wine town with half-timbered buildings, a great place to stretch your legs for a short time and to have a mid-day meal. See photo links below.

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Zum Goldenen Schlüssel on the Marktplatz is a good place for German food. They're closed on Wed and Thu but I think you arrive on either Monday or Friday, so they'll be open on both days you are there.

Marksburg Castle towers above the town and is open year-round for tours. While Burg Eltz is fancily-furnished and decorated, a Marksburg tour will show you the harsh life of medieval times.

https://www.marksburg.de/en/circuit/#/

On Day 1 you may or may not have time to include the 30-minute walk up (and back again) plus the 1-hour tour. So perhaps Braubach + Marksburg should be done on your 2nd day.

Assuming you're flying into Frankfurt, I recommend visiting a spa in Wiesbaden, staying the night there and taking the train to Koblenz in the morning, perhaps stopping at Rudesheim or somewhere else along the way. Or go and see the Chagall windows at St. Stephen's church across the river in Mainz. Wiesbaden has some high-end spas as well as public pools with sauna facilities. I found a soak in the hot pool to be just the refresh I needed after my transatlantic flight.

How about seeing Deutsches Eck in Koblenz, the "corner" where Rhine and Moselle join. You can reach Deutsches Eck from Koblenz Hbf by public transport.

If you do want to get out Koblenz, then I would suggest Limburg and Bad Ems.

Honestly, within striking distance of Koblenz there is a TON of stuff to see and do that qualifies as "Best of Germany" IMHO. Why the RS tours allocate so little time here baffles me. So you now have enough good suggestions from this forum to fill a week. What to leave out? I'd start with those things your GAS tour already includes according to Rick's itinerary...

Rick's GAS tour itinerary for Day 1 includes the following passage:

we'll take an orientation walking tour of Koblenz, learning about its
historical importance as a strategic base from Roman times through
WWII. After visiting the Deutches Eck, the symbolic heart of Germany
where the Mosel and Rhine rivers meet, we'll get acquainted over
dinner together.

Two days later, it suggests you might use your free time in Baden-Baden to...

...relax with a long soak before you enjoy dinner

Also... Threadwear mentions the Rhine cruise, which is indeed very scenic and often is the main goal many visitors to the Rhine Gorge. I could see fitting one in on the second day, probably in combination with a visit to G3rryCee's suggestion, Rüdesheim. The following schedule would accomodate both in your limited time and still get you back in time for your meet-up.

8:52 - 9:52: Train to Rüdesheim
See the sights, maybe take the cable car to the Niederwald Monument for a nice view and maybe a little walking.

12:10 - 13:30: Catch a round trip Castle-cruise (Burgenfahrt) with B-R cruises (food available) then get off at 13:30 in Bingen. After the cruise, Walk the Bingen waterfront promenade or get some refreshments there and enjoy the views.

14:30 - 15:27: Train back to Koblenz Hbf. Head to the BingenRhein Stadt train station (short walk) to catch a direct northbound train for the ride back.

These are great recommendations! Thank you! What do you think about visiting Schloss Stolstenfels and/or the cable car up to the Ehrenbreitstein fortress on the first day, and then taking the train down to Bacharach for some leisurely walks, sightseeing and lunch the second day? Not sure if there would be time to also go to St. Goar with its castle ruins before returning to Koblenz before 4:00. Or perhaps it would be better to train to St. Goar and see the castle ruins?

What do you think about visiting Schloss Stolstenfels and/or the cable
car up to the Ehrenbreitstein fortress on the first day, and then
taking the train down to Bacharach for some leisurely walks,o
sightseeing and lunch the second day?

Day 1: Neither the palace nor the fortress you mention - nor the Rheinfels ruins in St Goar - can compete with Marksburg, IMO.

Day 2: With an early start you would have time for both Bacharach and St. Goar's Rheinfels. Also, if you are skipping the cruise/Rüdesheim, and if you are using the train only as far south as Bacharach, you will miss out on a large portion of the castle/cliffside/vineyard scenery along the river, the part between Bacharach and Bingen. So my tip is to take an early train south all the way to Bingen first (8:03 - 8:42 for example) to take in that segment. Then catch the next train back to Bacharach (which takes around 10 minutes.) Then do as you suggested in Bacharach and St Goar.

If you do get to Festung Ehrenbreitstein sometime during the visit of Koblenz, apart from the its historical significance, ( assuming that is of interest to you), you will have from the fortress the view of Deutsches Eck, seeing it from above.

The monument on Deutsches Eck to the Emperor William I is a modest recreation from its Prussian original which sustained a good deal of damage in 1945 when US troops got to Koblenz.

All wonderful recommendations! Thank you! Leaning towards Braubach/Marksburg Castle on Day 1: Arrival day, and Bacharach and St. Goar/Rheinfels on Day 2 before meeting up with our tour at 4:00. Will need to work on logistics and energy level from and back to our hotel (Hotel Morjan) in Koblenz.

I imagine that we’ll see some of the greater length of the Rhine traveling to Koblenz on the train from the airport from FRA?

So you are confirming that you are in fact traveling from FRAon Day 1. The normal routing to Koblenz which DB provides (via Mainz) takes you through Bingen and Bacharach (along the west bank) on the way; you'll see Rhine most of the way once your train crosses the Rhine on the approach to Mainz.

Yes, your first train trip will show you the Bingen > Bacharach segment, so there's no need to include Bingen in the Day 2 outing. This outing will also use the west bank railway (if you do it.)

The only time you will be on the EAST bank is during the short outing from Koblenz to Braubach (if you do that, and if you travel only that far south.) If you wish to travel the rest of the east bank (a nice way to see the gorge from a different perspective) it is possible to change your Day 1 routing for FRA > Koblenz. To find schedules for this routing, simply input "Wiesbaden" as a stopover with 00:00 stopover time. You will need to make one change of train minimum on this journey if you make this choice; direct trains on this route are not possible from FRA.

If you take the standard routing along the west bank on Day 1... some journeys are direct, others require one or more changes. I imagine the direct ones will suit you better, so be observant when choosing your travel schedule if this is important.

I like what's been suggested, but I'll recommend you go to Bad Homburg. Stay at the Steigenburger.

Concur w/ Russ regarding Marksburg Castle. We visited this year (late audible over Burg Eltz) and it's a genuine high-perched real castle. Canons, torture room, . We had planned to visit/eat lunch in Braubach but it rained that day (downpour when the tour was ending) so we made our way to Rudesheim instead.

We stayed in Rudesheim, which I'd recommend as well. The seilbahn (cable cars) was a highlight for me. Very serene.

Bacharach and St. Goar/Rheinfels on Day 2

If you're in St. Goar for lunch, I'd highly recommend Zur Krone right near the train station.