My husband had an extra day off, the car just had an oil change, so why not a little road trip? We picked Mainz because aside from passing through the train station many times we hadn't been there and because my husband is a geek who wanted to see the Antique Roman Ship Musem.
I wasn't expecting a lot from Mainz, knowing that it was heavily bombed in WWII, but large parts of the old town have been nicely rebuilt, obviously a few buildings escaped the bombs, and most importantly, the Dom. For centuries 1000+ Cathedral was the power center one of Europe's most powerful rulers, an Elector and King-Maker, the Prince-Bishop of Mainz. If you're a Holy Roman Empire History nerd (or just a Roman history nerd) Mainz requires a stop. Along with other cities currently overlooked on many tourist itineraries like Worms and Speyer, in the early Middle Ages, Mainz had outsized influence.
One annoying aspect of the city is that due to construction, public transit between the sites of interest was pretty much non-existant. We stayed at the Hotel Hammer across from the train station which had pleasant service, a nice shower, and most importantly, air conditioning (highs of 35c during our visit!) We walked a LOT. We did use a "short ticket" to go from the HBF to the "Roemischer Theater" train station, on the regional train no less, for dinner, but other than that it was always as fast to just walk as to take the bus/tram so we did the latter.
However there were a lot of points of interest. In addition to the Dom and the Dom Museum - the latter of which could be enjoyed much more by a guided tour if you're not someone who does research on their own - you have the excellent Gutenberg Museum with priceless Gutenberg bibles and good exhibits in English on the two main floors (and presumably excellent exhibits in German on the 3rd and 4th floors) which is a bucket-list destination for any journalist, artist, or historian. A few sites are free, including the aforementioned Roman Ship Museum, housed in a old Locomotive repair house showing actual Roman ships discovered in the 1970s when a Hilton was being built, and reconstructions based on those finds and a temple of Isis underneath - and I'm not kidding here - a shopping mall.
We also visited St. Stephens, famous Marc Chagall's designs for it's windows that were destroyed in WWII. It's up a hill and I was fairly annoyed at the climb in the heat but it was far worth it once we got there - being bathed in ethereal blue light, with Chagall's universal take on Torah legends - I haven't felt so humbled artistically in a church since I was at Sagrada Familia in Barcelona. It was a unique and amazing experience. I'd seen Chagall windows at Reims which was also amazing but having an entire church be dominated by Chagall's blue light was something else entirely.
In addition to all this culture we had fun of course. Best currywurst of my life at Best Worscht in Town - using a scale of heat and adding different seasonings, we went with only a "B" level heat and it was enough for us (and we like spicy). We had an excellent dinner at Weinstube Loesch, where I had veal tafelspitz with gruener sauce and my husband had a schnitzel with fried eggs and onions on top. Service and price was good and portions were huge, like any good weinstube. We also enjoyed the city "beach" on the river for drinks and a surprising but delightful hipster biergarten at the city's Roman museum in the palace of the Prince Bishop.
Many tourists seem to bypass Mainz on their way to the tourist attractions of the Romantic Rhine, but with a world-class cathedral, a laid-back University city vibe, and cheap fantastic eats I think it's worth at least a day trip on your way up the Rhine, if not an overnight. (Way more than Koblenz or Cologne, seriously).