Toruń is a Gothic UNESCO old town on the Vistula, intact enough that the brick churches, walls, and granaries still feel medieval. It is Copernicus's birthplace (his house is a museum) and the city is famous for gingerbread baked here for centuries. The Leaning Tower, the Town Hall on the old market, and the river panorama are the usual circuit.
Unlike Warsaw or Kraków it is compact: you can cover the centre on foot in an afternoon. From Gdańsk it is a comfortable intercity hop into Kuyavia.