We are doing the Rick Steves 14 day trip next June. The tour begins in Cologne on a Tuesday afternoon. We will be flying into Frankfurt and due to using airlines miles will arrive on Saturday night. Would like to possibly find a small to mid size city on the Rhine to spend Sunday and Monday resting, exploring, and sightseeing before heading to Cologne on Tuesday morning. We aren’t entirely locked in to this plan so other suggestions are welcome as well. Thanks!
When we took the GAS tour in 2015 we arrived in Frankfurt. After arrival we took the train to St. Goar and stayed 3 nights at the Reinhotel. After checking in to the hotel, we had a visit with Gil, the owner, and he helped us figure out our itinerary. After lunch we visited Reinfels castle, before heading back into town for supper and early to bed. The next day we took the day river tour on the KD line. The boat dock was right outside our window. We rode up the river to Assmanhausen. You see all the cool castles along the Rhine river. In the evening we sat on our balcony and watched all the boat traffic going up and down the river. The second day we went down the Rhine river to Koblenz. We wandered around town and had some great gelato. The next morning we took the train to Trier, which was where our tour started. We really enjoyed our trips on the river. There were so many castles to see. We followed the information in Rick’s Germany book to know what was at each mile marker.
We are traveling to Cologne next week if Lufthansa doesn’t strike on Monday the 5th.
I'd suggest staying in Boppard at the Bellevue Hotel. As I learned here in the Forum, Boppard is a transportation center, and has many options for hotels and restaurants, and a great small town buzz. You can have many lovely dinners at cafes along the Rhine, and don't miss the chairlift!
Perhaps my Trip Report from our spring trip will have some helpful suggestions.
The previous posters describe Boppard and St. Goar, just 10-12 train minutes apart on the Rhine's "left" bank, very well. Both are well located for the outings you have in mind. Other towns on the same railway - like Oberwesel, Bacharach and Bingen - are easily accessed by train from these towns and offer good access to what the Rhine has to offer,. I've spent quite a few nights in both places and strongly recommend them both. On our most recent visit we spent 2 nights at the Rheinhotel St. Goar, as Janet did. There's no more scenic or relaxing spot in my Rhine Valley memory bank than the balcony that came with our room there. That said, both towns have hotel options with river-view rooms (including Boppard's Bellevue, where Pat stayed.) Wherever you stay, I would look first at rooms that face the river. In St. Goar, the Hotel Rheinfels, sometimes confused with the Rheinhotel St. Goar, is adjacent to the Rheinhotel and thus offers similar views. Both the Hotel Rheinfels and the Rheinhotel St. Goar offer breakfast with an excellent view of the river as well. This page on the Hotel Rheinfels from several years ago may be helpful in getting a feel for what it's like to stay in St. Goar.
St. Goar works if you are somewhat flexible about finding "a small to mid size city" - it's a very small town. Koblenz is the only true city (in the English sense of the word) nearby. Boppard is the next-closest thing to a city of all these Rhine towns and also the closest (15 train minutes) to Koblenz in the event you feel the need to be close to the kinds of city services Koblenz offers. I've stayed in Koblenz a couple of times as well, but Koblenz on the whole is quite modern and lacks the solid old-world atmosphere you'll find in the smaller places nearby.
Boppard is unusual when it comes to getting around by train. It's the only town in the immediate area where every overnight guest receives a "Guest Ticket" offering free local transport (train or bus) to nearby Rhine/Mosel towns:
St. Goar has one important transport advantage, the Loreley ferry. This ferry crossing shuttles back and forth all day long between the towns of St. Goar and St. Goarshausen, where there's a train station on the "right" side of the Rhine within a few minutes' walk. This combination makes for easy access to right-side towns like Braubach (Marksburg Castle) and Rüdesheim.