At present, the Krakow curia does not allow religious Roman Catholic ceremonies to be held outside of a church.
Civil outdoor ceremonies are possible in some communities around Krakow, but not in Krakow itself.

Generally, for most foreign citizens, it’s easier (less paperwork, money and trips to Poland) to get married in a registry office abroad, where you live, and only hold the religious ceremony over here. Remember, if your civil marriage is registered in Poland all the documents you might need for various institutions at home (act of marriage) will have to be obtained in Poland.