Visit the medieval part of Turku at Aboa Vetus museum. The city had a big fire in 1827, but its medieval stone cellars and paths can still be seen, cause these were buried under the ground when a garden was built. The oldest parts dating back to end of 14th century.
The information is easy to read (Finnish, English, Swedish). Kids have a play area after each stand. There is a modern cafe place in the museum house. I did not have time to go to the art museum, though.
Museum website Abo Vetus