For a first visit, the highlights are Frankfurt's old town with Römerberg and the cathedral, the Städel Museum, Kassel's Bergpark Wilhelmshöhe with the Hercules, the Niederwald Monument above Rüdesheim and Eberbach Abbey in the Rheingau.
Yes, if your layover is long enough to leave the airport. The S-Bahn takes about 10 to 12 minutes to Central Station, and from there the Römerberg, the cathedral and the Main Tower are close together. Check your return timing with the airport before you leave.
One full day covers Frankfurt, and a weekend lets you add the Rheingau or Darmstadt. To include Kassel, the Rhön and Limburg in the Lahn valley, plan four to five days.
The Senckenberg Natural History Museum with its dinosaurs, Frankfurt Zoo, the Roman fort at Saalburg and the Wasserkuppe with its summer toboggan run and glider museum. Kids under 12 get into the Städel for free.
From May 1 to October 3 on Wednesdays, Sundays and Hessian public holidays. The show starts at 2:30 p.m. at the cascades below the Hercules and ends at 3:45 p.m. with the Great Fountain. The 2026 season is over; the next one starts in May 2027.
Frankfurt's Römerberg, New Old Town and Iron Footbridge, Kassel's Bergpark Wilhelmshöhe, the Niederwald Monument and the grounds of the Mathildenhöhe are all free. The Emperors' Hall in the Römer is usually free as well.