I think the advice from nummer sechs is safe and sound. Frankfurt is a difficult city to fall in love with and requires time, Mainz and Wiesbaden are much "easier" for a first time visitor to savour.

In January, I would try some hot soup ( peas, lentils etc. ) with meat/sausage and mustard or winter veggies with a Frikadelle ( minced meat steak ) for a hearty lunch.