Frankfurt's central location and excellent train network make it a strong base for day trips. Within an hour by train you can be in the Rhine valley at Rüdesheim or Mainz, the spa-elegance of Wiesbaden, or the picturesque university town of Heidelberg an hour to the south. Darmstadt, half an hour by S-Bahn, has the Hochzeitsturm and Mathildenhöhe — the centre of the Jugendstil movement in Germany. Marburg an hour and a half north is a perfectly preserved medieval university town with a castle on the hill and one of Germany's most beautiful Gothic churches.
For a longer day, the Romantic Rhine — the World Heritage stretch between Bingen and Koblenz, with the Loreley rock and a string of hilltop castles — is best done as a cruise: take the train to Rüdesheim, board a KD river boat, and disembark at St. Goarshausen for the train back. Allow a full day. Trier, Germany's oldest city, is two hours away on the ICE and a long day trip; better as an overnight if you can manage it.
