Hotel Alte Post, Gustavsburg
I don't like to stray too far from FRA after the long flight from California, but I'm not fond of airport hotels generally, so I've stayed in Frankfurt or in nearby Mainz on previous visits - and on my most recent trip too. But I found this place in Gustavsburg (which is actually part of Mainz) for my final night in Germany this time - and I only wish I'd found it sooner. It's a little closer to FRA than the Mainz Hbf station - 20 minutes on the direct S-Bahn 8 train from FRA airport's regional train station, with service every 30 minutes. Gustavsburg's station looks to be in the middle of nowhere - and it's small enough that it would be better called a stop than a station - expect no services here other than a ticket machine and a WC. But it's a flat, easy, nearly traffic-free walk to the Alte Post from the station exit, I'd say 2 minutes max, to the very modest-looking front entry.
https://www.alte-post-garni.de/welcome/
But this place is a gem wrapped in a plain cardboard box. My roomy "comfort"-level single with bath, balcony, workplace/desk + chair and settee was done with finely-kept wood doors and trim, the kind they just don't make anymore, and every square inch had clearly been immaculately maintained for decades. The room was spotless. The rest of the hotel was no different. I felt transported to some other era. It's a family-run place that shows intense pride of ownership - and that includes the personal interactions with very welcoming and helpful family-member/manager Anja, who should be giving lessons to hoteliers everywhere. Breakfast was terrific and included in the price of 93 Euros.
Gustavsburg itself isn't much. But with a walk of maybe 2-3 blocks to the main street (Darmstaedter Landstrasse) I found a Penny-Markt, a steakhouse, an ice cream parlor, a bar... If you plan to eat in town, check the options out in advance. The better choice would probably be to take the S-Bahn across the train trestle that spans the Rhine River into Mainz - it takes less time to get there - only 4 minutes to the next stop, which is Mainz Roman Theatre station - than my walk took to the Penny Markt. And the MRT station is adjacent to Mainz's old town zone, which is peppered with eateries of all kinds.
One more nice thing about Gustavsburg is the huge park in between the station and the Rhine River which the locals seem to use for dog-walking, bike rides, and similar. I took the trail through this park for about 1-1.5 km out to the point where the Rhine and Main rivers meet ups (called the Mainspitze.) Nice views of Mainz across the river from there.