I had to overnight at FRA last June, and instead of the usual shlep to Frankfurt, I decided to make use of my Hyatt status and stayed here. I had similar reaction to the hotel as Lucky (and had the same welcome amenity and the same Asian flight attendant corps in the morning)! It is a short walk from the S-bahn to the hotel, though if one has lots of bags than a taxi is definitely recommended.
While I'm sure the Christkindlmarkt was fine, when I was in Mainz, the printers' festival was going on. (Gutenburg was born in Mainz and spent much of his life there, hence the focus on printing.) This was entirely unplanned, and also amusing when I came across the dunking of a bunch of young kids in a big vat of water on a big stage and didn't know what to make of it. I chalked it up to it being another strange German tradition. I later found out that these were printing apprentices being "baptized" into the printing industry as part of the annual St. John's Night festivities, as shown here:
http://www.mainz.de/WGAPublisher/online/html/default/mkuz-5tveyh.en.html