As promised, here is a bit more information about Frankfurt and the surrounding area. Although Frankfurt is relatively small as cities go with a population of only ca 670,000 there is plenty to see and do.
Frankfurt is a modern city with only a small historical district around the cities town hall (Roemer). This is the area in which the annual Christmas market takes place.
Nearby is the main shopping area called Zeil . The street is a pedestrian-only area and is bordered by two large plazas, Hauptwache in the west and Konstablerwache in the east. There is a mixture of department stores and smaller shops in this area. Near Hauptwache is a small indoor shopping centre called Zeilgallerie. Not only can some of the more quirky stores be found there, but they have a viewing platform on the top which will give you some wonderful views over the city skyline.
From a historical point of view Saint Bartholomeus' Cathedral (a beautiful Gothic cathedral), the Old Opera House and Saint Paul's Church are well worth a look. Saint Paul's Church was the seat of the first democratically elected parliament in Germany.
Frankfurt has more than its fair share of museums ranging from art, history, film to communication. The most famous museums are the Städel, which showcases European paintings spanning 7 centuries, and Goethe Haus, the birth place of the famous German author Johann Wolfgang Goethe. Both are worth a look. My favourite museum is the Senckenberg Museum, which is Frankfurt's Natural History Museum.
Another place that is worth a visit is the zoo.
The local drink is Apfelwein (apple wine) which is either drunk on its own or mixed with sparkling water or lemonade. Something worth doing is a city tour on the Ebbelwei-Express (Apple Wine Express). The trip in a historical tram takes about 1 hour and passes many landmarks. It is hop on hop off and a glass of apple wine and a pretzel is served during the trip.
Frankfurt is located on the River Main, which merges with the River Rhein nearby. There are regular river cruises lasting from 1 hour to a whole day. Again, this is well worth doing, especially river cruises that last half a day or longer.
If you get bored of Frankfurt, there are two places that I would recommend for a visit. My favourite city in Germany is Mainz, which is only about 30 minutes by commuter train. Mainz has a wonderful historical quarter around the cathedral. The cathedral itself is well worth a visit. However, it is kind of overshadowed by St. Stephen's Church nearby. This church has a full set of stained glass windows by Marc Chagall and is absolutely breathtaking.
Another must see in Mainz is the Gutenberg Museum. Johannes Gutenberg invented the printing press with movable types. In the basement is a replica of the printing press. On the other floors are books and manuscripts from around the world spanning 4 centuries.
Apart from the cultural stuff, it is nice just walking through the historical streets or along the river bank.
Going by train about 30 minutes in the other direction from Frankfurt, you will find the spa town of Bad Homburg. The formal gardens surrounding the springs are stunning and a visit to the castle and the nearby Church of the Redeemer (Erlöserkirche) is also highly recommended.
Corinna