Frankfurt itself has plenty to see besides the Christmas market. Only Berlin has more museums and there are so many historical sites here that Rick has chosen NOT to include in his guide books, that many people believe there is nothing here but skyscrapers. Why he wants to present Frankfurt this way, instead of featuring the extensive history and the many historical sites and medieval churches is a mystery.
Anyway, using Frankfurt as a hub to visit lots of other towns and their Christmas markets is wise. Easy access to the many trains, using regional or local transport tickets from www.rmv.de They have expanded the Christmas market here and with the 200 decorated stands, it is really photogenic and offers something for everyone. It is also one of the oldest, going back to 1393. There are smaller markets that are just food and drink at the Thurn & Taxis palace, Gibson City Xmas by the Alte Oper, and the City Alm Xmas on top of a parking garage, Paradis Platz in Sachsenhausen and a bit larger one in the City Forest at the Goethe Turm that also has handmade items and some rides for the kids.
My recommendations for markets to visit when using Frankfurt as a hub: Mainz, Limburg, Wiesbaden, Marburg, Seligenstadt, Michelstadt, Rüdesheim, Fulda, Heidelberg, Worms. Depending on the weekend you are here because some markets are only one weekend - Büdingen, Idstein, Hessen Park. A bit farther away and with possibly a hotel change - Koblenz, Ravenna Gorge (stay in Hinterzarten and go on the torch walk in the evening and shuttle back to the town after the market.)
Central hotels in Frankfurt - Motel One is the top location, but the Hilton, Ruby, Moxy, and Flemings are good too.
The Winter Lights in the Palmengarten have just begun and that is a great way to spend an evening if you are marketed-out. There is the Christmas Garden at the stadium, but after going to the one in Koblenz (see my trip report) I am now spoiled, as that one tops anything else.