They loved history/science museums, public transportation, trying new foods, castles, and anything outdoors on our last trip. My son has celiac so we need to go somewhere we can manage gluten free.
I may be a bit biased but I think Spain fits the bill! History in Spain is deep and rich spanning millennia and many cultures, what better lesson to learn, especially these days, than of when Christians, Jews, and Muslims lived in relative harmony during the time of Al-Andalus.
Spain has the most developed high speed train network in Europe, I think it will be quite the experience zipping around the Spanish countryside in an ultra modern sleek bullet train going over 130 miles an hour.
Spain has arguably the most diverse and interesting food culture again it gets from the rich past. We are also very gluten free conscious will many products labels indicating "Sin Gluten" (without gluten). 95% of our cuisine is naturally gluten free too, think paella, jamon, patatas bravas, tortilla, various pork and seafood dishes.
Castles we have by the boatload as during the Reconquista there were castles built each time the frontier moved. Walt Disney used the Álcazar of Segovia and a few other Spanish castles as inspirations in his Disney Films. Here is a video on some good ones but there are plenty more - https://youtu.be/dbDbGgO6Ps4?si=9UNC_i3ZAJjCJLk1
Also regarding outdoors, Spain has the highest level of biodiversity in Europe, deserts to rainforests and everything in between. Spain's national parks and nature reserves, such as Doñana National Park and the Aigüestortes National Park, showcase this incredible diversity. In the north, the Picos de Europa offers breathtaking mountain scenery, lush meadows, and deep, mysterious caves like the Alps mixed with Ireland. Spain's coastal areas, such as the Costa Brava and the Balearic Islands, provide habitats for a wealth of marine life, and also have many coastal hiking paths like the Camino de Ronda. From the Iberian wolf to the Cantabrian brown bear, the country is a sanctuary for a variety of endemic species that have long since died out in other parts of Europe.