Everything obviously depends on how much time you have and what your interests are. Heading from the south, I would recommend going to Cracow and Wieliczka for a few days. If time allows Zakopane is always worth seeing. There is Nazi Concentration Camp in Auchwitz - Oswiecim, and I believe this place everybody needs to see. Than travelling up, I would stay for 2 days in the capital of Poland--Warsaw and than I would go to 3-city. You are going to love it! Gdansk, Gdynia, Sopot by Baltic see.