For a curious traveler, Wiesbaden offers something that most German cities do not: a beautiful old town. In contrast to its neighboring cities of Mainz and Frankfurt, Wiesbaden offers an entire city of specacular vistas and 19th century atmosphere.
Frankfurt is not an especially pretty city. If you find yourself trapped here, asking, "What can I do?" the best answer is to go somewhere else. I aim to give you a list of places to visit within roughly 60-90 minutes by regional train from Frankfurt Central Station. The destination needed to have a direct or otherwise easy train connection to qualify. Likewise, it also requires an old town with enough things to do to fill at least half of a day.
The Via Regia is an ancient trade route that stretched across Central Europe, connecting Krakow and Paris, with major stops in Frankfurt and Leipzig and several divergent paths. Part of the road has been preserved near the town of Fulda as a result of its use as a military highway in the 18th and 19th centuries and its abandonment in the 20th. This hike walks along its old route, and occasionally you may still see some original pavement.
For an easy hike across some of the nicer farmland north of Frankfurt, there is the Monastery of Ilbenstadt. The monastery complex is mostly preserved and features a remarkable late Staufen Romanesque church and a Rococo gatehouse, hinting at the centuries of prosperity enjoyed by the monks here. Behind the monastery is a nicely preserved agricultural complex from the 19th century.
The Red Moor is an ancient wetland in the highlands of the Rhön. Though much of it was drained or harvested for peat, today, it is a nice glimpse of the primeval landscapes of Europe. Including ancient deep ponds or "eyes of the moor," which served as cult sites for worship through to the end of the middle ages.