Mainz - hotel recommendations

What is your budget? People need to know how much you want to spend in order to make suitable recommendations.

Something less than 100 euros a night which includes breakfast and wifi would be great.

We stayed at GuestHouse Mainz. I don't think it's less than $100 Euros a night, but it's not super expensive, either. The rooms are small, but well-appointed. Breakfast is included. The location is good--fairly quiet and in-between the train station and the historical centre.

The Ibis Mainz has an excellent location near the old town zone and the Mainz Römisches Theater train station (one stop closer to FRA airport than the main station.) There's a "Breakfast and Surf" rate which includes breakfast (natch) and premium wifi.

If you will be making certain outings by train, you might wish to be located near the Hauptbahnhof (main station) instead; there's a cluster of station-convenient hotels there which includes the Königshof, a popular choice that is likely in your range even if it's not as convenient to the Dom, the Marktplatz and some of Mainz's other main attractions.

I've spent a night on three occasions in a hotel across the plaza from the Hauptbahnhof in Mainz, twice (1988, 2000) in Schottenhof and once (2008) at Königshof. In 2000, I stayed at Schottenhof because that's where I had stayed in 1988, but I walked past Königshof on the way to the Hbf and thought it looked appealing (less walking distance, nice looking place). I stayed in Königshof the next time and would again. I particularly liked the breakfast at Königshof; although I did not care to partake of it, there was a side table with American style breakfast dishes.

I stayed in this area because of its proximity to the train station and the S-Bahn to FRA for an early flight.

Go for the Ibis, as it will have AC, which you may very well need in July.
Mercure is also good.
Check other hotels with www.hrs.com so you can use the filters like location, price, or AC

I stayed at the Ibis on Russ's recommendation and earlier at the Königshof at Lee's suggestion. Both were good. I liked the area around the Ibis better and their breakfast. Königshof charged me in dollars at an exchange rate that was more favorable to them than to me. I would likely choose the Ibis again.