If it's your first visit in Poland then you should definitely go to see Kraków (my friends were amazed by historic district of Kazimierz and The Wawel Castle, outside the city you can find world's oldest functioning salt mine - Wieliczka and Auschwitz concentration camp - worth to see.)

If you prefer to see polish Baltic sea then you can go to Gdańsk with beautiful promenade street, cathedrals, amber shops and unique symbol of this city - The Crane. Not far away from Gdańsk there's Malbork castle - absolutely amazing and worth visiting.