Which is your plan to cross Poland?
Krakow, Wroclaw and Warszawa are places to visit.
If you are interested on WW2 sites the Wolfsschanze next to Ketrzyn (Rastenburg) might be a place to see. Stettin and Gdansk are most likely more en route for a Baltic trip and also worth a visit.
Do not underestimate the distances in Poland, especially at regions without highway connections.
