Frombork is a historic Prussian town by the Vistula Lagoon — here Nicolaus Copernicus wrote "On the Revolutions of the Celestial Spheres" (1543) and here he is buried.
Cathedral Hill: Gothic cathedral (1342) with Copernicus's grave (identified in 2005 through DNA analysis), bishops' palace, Copernicus Tower (with telescope for visitors), planetarium.
Copernicus Museum with original manuscripts. Astronomical shows every 30 minutes. Daily antique fairs.
100 km from Olsztyn. Open daily 9:00-17:00. Combined ticket for all sites 30 PLN.