3. Germany's Frankfurt and Nürnberg

Skyscraper-studded Frankfurt — with so much commerce it's nicknamed "Bankfurt" — has a delightful-to-explore old center. We'll also visit Nürnberg, a capital for both the First Reich (the Holy Roman Empire) and the Third. From its towering castle and playful fountains to its Nazi Documentation Center and maze of underground bomb shelters, Nürnberg is a fascinating study in contrasts.