I have the opportunity to join my wife in Frankfurt in mid January at end of business trip. Looks like we'll have Saturday, Sunday and Monday available to sightsee. Can anyone recommend things to do there or should we try another area? Worried that things won't be open in the off season. Thanks for your input!
Frankfurt is a metropolis that doesn't close.
For a smaller city look into Mainz, just 25 minutes from FRA airport by direct train. Mainz has a small but pleasant pre-war old-town zone and a large pedestrians-only zone (see pink streets near the cathedral?) as well as a good number of sights.)
https://c1.staticflickr.com/9/8517/8466997951_748e0b61f0_b.jpg
http://www.tripadvisor.com/Attractions-g187393-Activities-Mainz_Rhineland_Palatinate.html
It's easy to make a day trip by train from Mainz into Frankfurt if Frankfurt interests you. It's also easy to day trip to other places from Mainz - Wiesbaden and Rüdesheim are a couple of Rhine River options. Passenger rains continue beyond Rüdesheim along the scenic part of the Rhine River Valley past castles and attractive villages to the city of Koblenz, where you could change trains and return to Mainz along the other side of the river. Here's the view near St. Goarshausen:
http://www.bahnbilder.de/1024/140-184-bei-stgoarshausen-23041988-533040.jpg
Middle Rhine Valley: http://whc.unesco.org/en/list/1066
Some of the seasonal restaurants along the Mittelrhein (if you consider a daytrip here) close for the colder times of the year. Some castle ruins may close when snow makes the approach impassible. That's about it. Everything in Frankfurt should remain open, including zoos and parks- the nearby open-air folk museum, Hessenpark, was open on the day I visited with several inches of snow on the ground.
Frankfurt has a plethora of museums to visit as well as many historical sites and churches. Including one of the oldest churches in Germany, the Justinus Church in the neighborhood of Höchst. It has a very rich Jewish history with many sites to visit. The more you tell us about what you like, the better suggestions I can make. Otherwise, I will send you off to the Leather Museum, when instead you wanted to see the Luther Bible in the Bible Museum. The Palmengarten is wonderful in the winter and they will be having their Winter Lights exhibit until 21 Jan. Bring your camera for this. The Farmers Markets take place all year, so visit the one on the Konstablerwache on Sat. Stop in the Klein Markt Halle too. Do visit the Karmeliter Cloister, as this is one of the gems of Frankfurt.
Hessen Park is always a treat as is the Saalburg Roman Fort. Smaller towns that are within an hour by train, that might be of interest are Eltville, Bad Homburg, Limburg, Marburg, Büdingen, Gelnhausen, Idstein, or Seligenstadt.