If you find yourself with time to kill in the hub city of Frankfurt, check out this blog post from Venere.com about it.http://www.venere.com/blog/24-hours-frankfurt/
Firstly, line 8 of the S-Bahn runs from the airport into the centre of the city in a mere fifteen minutes. This will be the best bet for short-term visitors since a taxi into town is so much more expensive and if you are not carrying too much luggage the S-Bahn is easily manageable. A good place to start is the Romerberg, the main central square until modern development expanded the city. Here the old town hall and other restored buildings give a glimpse of what Frankfurt was like in the fourteenth century. In close proximity to the Romerberg is the Frankfurter Dom, which to all intents and purposes serves as a Cathedral but as locals will always tell you is in fact not a cathedral. Whatever the specifics of its designation, the building is impressive and the view from the tower well worth the climb. Also short distances from the Romerberg are the History Museum and the Museum of Modern Art, both of which contain interesting and well presented exhibits.
For those less inclined towards museums various tours of the city are available which pack as much as possible into a short time. The best of these is the Ebbelwei express, which is a tram that circles the main landmarks of the city allowing visitors to jump on and off whenever they wish. Also fun are river cruises which run from the pontoon directly south of the Romerberg. Visitors should be careful that they are getting on a short city cruise and not a longer trip up or down river as those air connections could easily be lost.
A visit to Frankfurt is an opportunity to eat in a good restaurant and reduce your reliance on airline food. There are solid, traditional restaurants near the Romerberg but for sheer variety of cuisines head west towards the Bahnhofsviertel, where curry houses and falafel joints stand next to flamboyant Italian eateries. For yet more adventure cross the river to Sachsenhausen, which is a lively and colourful area of cafes and bars. Those of a nervous disposition should be warned that this is the centre of gay life in Frankfurt and unusual sights are here usual, but those with an evening or night to pass in the city will find this is the place to go. Most tastes will find something to keep them happy until whatever hours of the morning require them to catch the S-Bahn back to the airport.