The Emperor's New Clothes

Hold an empire title. Be naked. The game rewards you for this. The reliable route: replace the Byzantine or Holy Roman emperor with a Cynical piety-farmer, take the Theology focus, and once you've banked enough piety convert to Adamitism - the nudist faith whose adherents are, as required, naked. Slower alternatives the wiki notes: the Lunatic trait has a small chance of a nudity event, and a high-danger Pilgrimage towards Jerusalem can bump into Adamites in the Balkans or Anatolia with a convert option. NB the wiki flags it may not be available for ruler-designer characters, so playing it safe means taking over a historical emperor rather than designing one.