Frankfurt-am-Main, the fifth largest city in Germany, is not the most exciting city (to me, Berlin holds that title). However, its prominence as an important financial, communications, and transportation hub means that I have stopped by for a few times, in hourly or daily increments. It is a pleasant enough city for a short stay, as it is mostly modern with a few historical and reconstructed touches.
I highly recommend a trip to the viewing platform (Dachterrasse) atop the busy Zeilgalerie shopping center. It is free, but the fine views from the top are definitely worth more than that.
Quick Tips:
There are plenty of good museums in Frankfurt; many of them are part of the Museumsufer along the south bank of the Main River. Most of them are closed on Mondays, while some have free admission on Wednesdays. There is a flea market held every Saturday in this area during the warmer months.
The area just east of the Hauptbahnhof (central train station) is colorful but a bit seedy, as the red light district is located here. An amalgam of cheap foreign eateries and stores intermingle with sleazy sex shops. Once I was physically prevented from taking a photo by a man who basically did not want his building to be photographed. Use reasonable caution if you are walking between the station and the old town.
Best Way To Get Around:
The beautiful thing about visiting central Frankfurt is its proximity to the Flughafen (airport) Rhein-Main. It is a mere 12 minutes by train from the airport to the Hauptbahnhof, with a usual run of four trains every hour. You can buy an all-day pass just outside the airport that is valid on local trains, trams, and subways, all excellent cogs of the public transportation system. The cost of this pass is only a few cents more than a typical round-trip ticket price.
The central part of town is eminently walkable, with a nice variety of pedestrian streets, old plazas, and scenic river walks.