Originally Posted by AlienInTheFatherland

Like many cities, the area around the main train station in Frankfurt isn't the nicest. Although I would not say it is unsafe.

If you stay in Frankfurt then go for somewhere on/near the river (Main).

Also consider a short hop to Mainz or Wiesbaden (both reachable on suburban trains from the airport's regional station). These are two cities facing each other on the opposite sides of the Rhine. They both have a very good selection of hotels.

Then take the train from Mainz to Cologne (follows the Rhine valley most of the way - sit on the right side to get the best views).

Overnight in Cologne (one of Germany's few cities with a population over one million and bestowed with plenty of very old churches, museums, etc.) and then perhaps even consider going on to Paris directly from there by train