Looking for suggestions for day trips (or even overnight) and am travelling with two 14 year olds. I've thought about Amsterdam (overnight). Suggestions?
Amsterdam is just too far from Frankfurt for even an overnight day trip. Pick something closer.
Heidelberg would be my first choice. Cologne is a possibility, but that is still some distance from Frankfurt. You could visit Wiesbaden or Mainz. Rothenberg on the Tauber is another one, suggest a bus tour, since train connections are not the best.
What kind of things are they interested in? Castles, Romans, Ships, Trains, Cars, WW2 stuff.
Have you asked them what they would like to see?
There are any number of cool destinations within just an hour of Frankfurt including the Rhine with all of its' castles. For Roman stuff go up to the Saalburg which is near Bad Homburg, You can include a trip to Hessen Park Open Air museum on the same bus/train ticket for the day.
Train to Bingen (Rhein)
KD Line cruise to Koblenz
You could overnight here, then next day train up the Mosel to Trier, before finally heading back to Frankfurt.
They won't give me any ideas other than they want to do everything they can. I like the idea of a day cruise and WW2 and Romans. And of course we have to see a few castles!