2-days within an hour of international airport - Does Mainz or Heidelberg look like a good option?

Mainz is a good location, so are Wiesbaden, Darmstadt, Heidelberg and smaller Eltville with wine region Rheingau.

I am not sure which specific "vibe" you are looking for. If it is the scientific sniff you might like Darmstadt with technical university, ESA sat control center and the EXPERIMINTA ScienceCenter. There is a direct bus connection called AirLiner between Darmstadt and Frankfurt airport (link to brochure).

Thank you! i will have another look at Heidelberg & Wiesbaden. Will save Darmstadt for a future trip with my science-centric husband. Eltville looks enchanting! I think we'd like to have a few different museums to visit, thus the thought behind Mainz. PS, In another post you mentioned Speyer, which is a bit further by train but wow, looks like an amazing town and the cathedral would be a plus for me. I will run that one by my friend to see what she thinks.

Eltville is well connected to public transport RMV, so every museum in the mentioned cities plus Frankfurt is in reach.

Mainz is a pretty big city with about 225,000 population, so not a small town at all.

Eltville is my favorite town on the Rhein. Head up to Eberbach Monastery, visit Rüdesheim, or visit Mainz, Wiesbaden or Frankfurt. In Rüdesheim, go to Siegfrieds Musical Cabinet. It is very unique.

If you decide to stay in Mainz I recommend staying at the Hyatt Regency which is on the river and built into some of the old city fortifications.

Don't miss visiting the Isis Temple. https://roemisches-mainz.de/

Eat here: https://www.heiliggeist.de/

Wiesbaden is just across the Rhine from Mainz so can easily be visited and not really a ‘day trip’

Thanks everyone. I love travel planning, so being asked by my English friend to 'show her something' of Germany, I knew the Forum would help. We're still thinking Mainz would be best at this point, it's large enough to have a variety of places to eat & museums as well as some Roman ruins. I like the idea of visiting Wiesbaden if she's interested.

Great idea about the Hyatt - I will take a look at my points & free nights to see if we can swing it, otherwise frankly the Ibis looks clean and easy.

The location of the Ibis is more convenient than Mainz's main-station hotels for exploring Mainz on foot. From there it's an easy walk up Neutorstrasse to Augustinerstrasse, the pedestrian artery that runs through the old-town zone toward the Marktplatz and the Dom. And just one block away from the Ibis is a secondary rail station - the Mainz Roemisches Theater station (where the "S" is at the bottom of the map that follows) which provides direct trains to Mainz Hbf and to FRA airport. (The numbers on the map are NOT necessarily the most prominent things to see in Mainz unless you are into Byzantine-era war cultures. You will find this page more useful for things to see/do in Mainz.)

While the Hyatt isn't too far from the old-town, it's a longer distance to either or the two rail stations.

Map: https://grk-byzanz-kriegskulturen.uni-mainz.de/files/2021/06/Stadtrundgang_Karte.jpg

Hey thanks everyone, so many good ideas! I'm working on a Google map for the area, and it does look like regardless of the actual size of Mainz, the old town and most important monuments are all in a small area, and yes closest to the Ibis hotel, good to know! I appreciated the map, good to download ahead of time, thanks.

Any suggestions on not too crowded Weinstubbe (wine bars) to have some decent food along with a few good local glasses of wine? A pleasant atmosphere with good food is really enough. (During my university days there, I would only have tasted the cheapest wines.) I've checked out wine walks & wine tastings, frankly we just don't drink enough.

H2 Hotel in Mainz is often to get for a cheap night and breakfast (I like it!) is included. It opened one or two years, ago. Very nice hotel. Near the River Rhine for a nice walk and I walk always to the City, Oldtown, Main Station. This is my favourit hotel for visiting Mainz

I admit to being guilty of looking at Heidelberg now as well.... Decades ago while living in Germany I was an underpaid tour guide to American high school groups there, before taking the train back to Tubingen exhausted to study. Looking at Heidelberg again through new eyes, it looks like it might meet the criteria of relaxing / not too jammed / university town vibe with plenty to wander, sites, churches & yes a castle. Looks like 1.3 hours from FRA. Thanks!