Central Germany

How about Bacharach on the Rhine River? It's on a rail line easily reached from Frankfurt Airport in about 45 minutes. It's a very quiet place with walking trails along the river and very good bed and breakfasts. You can catch the train up to Koblenz and Cologne. And you can catch a tour boat from there on the river.

Schwabish Hall, Nordlingen

Wurzburg or go north on the Rhine.

Maybe it's too far east, but consider Weimar. Daytrip to Erfurt for EGAPark, among other outdoor activities.

The places mentioned so far are either not east of Frankfurt or not smallish. I can think of two that fit your criteria better. They are located at the northern and southern edge of the Spessart National Park and offer a lot of outdoor activities in a charming landscape: Bad Orb and Lohr am Main.

East of Frankfurt? Try Büdingen or Gelnhausen. Lovely, medieval, small, hiking trails. Easy to reach by train from Frankfurt.

Try Miltenberg.

https://www.youtube.com/watch?v=N9nwkeiWgcU
https://www.miltenberg.info/en/ (TI office will have trail and other visitor information)
https://www.miltenberg.info/wp-content/uploads/2014/08/miltenberg_wandern_03.jpg
https://hastingshouse.typepad.com/hastings_house_us/2008/05/miltenberg-germ.html

Franken Wein Region. Many small Wein Towns, Volkach, Prichsenstadt, Iphofen etc. Main River Valley. Lot's of hiking and biking opportunities. Family friendly.

Central Germany...where? How east? Or, even northeast, or still within Hessen?

I recall visiting Bad Brückenau years ago. Its a nice area. In 1816, following the Napoleonic Wars, Brückenau became part of the Kingdom of Bavaria. King Ludwig I of Bavaria had a hand in renovating and building some of the buildings by the Bath/Park. He used it as a summer residence. He also got tied up into an affair here that caused him and the monarchy problems. Interesting reading and history...
https://www.bad-brueckenau.de/vorschaltseite/index_en.html

The wine region around Wurzburg is an easy goal. Lots of small towns around there (I like Kitzingen). But if you take the train to Nuremberg and then to Amberg you find a nice, smallish city with very few tourists and lots of character. There's a huge nature park just south of town and lots of local paths. And the small towns that pop up every 3-4 km are on nobody's tourist map, so they can be a lot of fun, even if all you do is grab lunch in the local gasthaus.

Hi everyone,

thank you so much for all your replies, it is much appreciated, it's given me plenty of great ideas.

Best wishes,
Brad

I want to second SLA's suggestion of Lohr am Main. My SO and I spent 9 wonderful nights in Lohr a little over two years ago. The town, on the Main river, is just a quaint little German down with a main street loaded with Fachwerk buildings. Other than a palace connected with the fairy tale of Snow White, it offer few touristy attractions, but is a nice place to visit, shop, dine, relax, and it's good base to explore nearby Würzburg, Bamberg, or even Frankfurt a.M.

If you travel with public transportation and stay there long enough to be there over a weekend, the buses don't run between town and the Bahnhof between Saturday noon and Monday morning. Either plan on 1) staying in town and enjoying the town and touring it on the weekend, or 2) staying near the Bahnhof and making day trips to other places on the weekend, or 3) staying between the town and Bahnhof, where you can do both. There is one Ferienwohnung (Hoffmann) and one Gasthof (Küferstube) that fit the third criterium.

I picked Lohr in 2017 because it was close to Würzburg for day trips and close to FRA for my trip home, but I was pleasantly surprised by the town itself.

BTW, if you go there, find accommodations using the town website (hotels or apartments) or Google Maps and book directly with the places. There are some accommodations in Lohr listed on booking websites, but those are only the most overpriced third of the properties in town, so by only using booking websites, you would be limiting your options. And if they really are what you want, those properties are also shown on the town website.

Rothenberg od der Tauber is a untouched walled city that may be on of the prettiest cities in Germany

Untouched? 40% of R'burg was destroyed in WW II. But the rebuild was done nicely and it is very pretty.

Rothenburg is far from untouched and certainly not the only pretty, quaint, cute town in Germany. You mustn't believe everything that Rick says, cause lots of times, it isn't correct, nor true.

Check out towns like Büdingen, Celle, Gelnhausen, Marburg, Limburg, or any town on the Half-Timbered Route. https://www.deutsche-fachwerkstrasse.de/en/Homepage.html

Lohr makes a good base for a day trip to Rothenburg. Just over 2 hrs each way. You can use a Bayern-Ticket. If you want to leave earlier than 9 AM on a workday, buy a single trip ticket to Karlstadt for €5,80 in addition to the Bayern-Ticket. Leave on the 8:42 from Lohr. The single trip ticket covers you to Karlstadt at 9:05; the Bayern-Ticket covers the rest of the way to Rothenburg at 10:50 and back that evening.

You really should tell us more. Are you already going to be in "central Germany"? Where? Or are you flying in from elsewhere? Do you already have an arrival airport, or is that pending?

Assuming you are arriving at FRA, I have a suggestion that is actually mostly a little south of Frankfurt, in the northern Black Forest. Bad Herrenalb is a small town about 40 minutes by streetcar from Karlsruhe Hbf. The town itself is nestled in a steep wooded valley surrounded by hiking trails. I've seen people rock climbing on the volcanic formations at the edge of town. It has a Therme (spa) with a family side with a big swimming pools and an adult (nude) side with saunas and pools.

Using a Konus Card, which you get with the Kurtax you pay with you accommodations, you can go by bus, train, and streetcar to surrounding towns. You can visit larger Karlsruhe and Baden Baden or go by bus to other great spa towns, Bad Wildbad and Freudenstadt. When I was there some years ago, I also visited the Fachwerk town of Calw, which was Herman Hesse's home town.

Mainfranken wine region, somewhere between Aschaffenburg and Würzburg.