If you only have a single day in Frankfurt, you should focus on those sights north of the river.
Start out by walking north along the iconic Eiserner Steg (Iron Bridge) en route to the Altstadt. Walk past the Customs Tower and Wertheim House and into the Historisches Museum, where you can spend an hour or two learning about the history of Frankfurt and the surrounding area.
From here, head into the main Römerberg square to see sights such as the Ostzeile (a row of six picturesque half-timbered houses), the Alte Nikolaikirche (a pretty, 13th-century Gothic church) and the impressive Gerechtigkeitsbrunnen (the Fountain of Justice). Then, stop for a swift lunch in one of the excellent, if expensive, cafés and restaurants lining each edge of the square.
After lunch, turn just off the square to look around the impressive St. Bartholomew’s Cathedral — best known as simply “The Cathedral” and one of Germany’s most significant Houses of God — then head north to take a look around Goethe House. The somewhat unassuming birthplace of the country’s most renowned author and poet. If you still have energy, you can then head to the Jüdisches Museum on Untermainkai, which documents 850 years of Jewish history in Frankfurt.
In the evening, reward your efforts with a meal in the traditional Frankfurt eatery Max on One Grillroom, followed by an unforgettable evening of tasty cocktails, wonderful live music and sensational city views in the popular 22nd Lounge & Bar.
If you’d like to see a small slice of life south of the river, head to one of the quaint cider taverns in the old suburb of Sachsenhausen.