Aboa Vetus et Ars Nova pairs two museums in one building: medieval Turku below ground and contemporary art above it. The Aboa Vetus side is the standout, with excavated stone houses and old street levels that let you walk through the buried city rather than just read about it.
The mix works better than it sounds. After Turku Cathedral and the castle, this is one of the best places to understand the city as a lived-in medieval town, while the Ars Nova galleries add a sharp modern contrast.