The Bochnia salt mine (located around 20km from Kraków) is one of the oldest mines in Europe. It's also the only one in Poland to have an underground sanatorium, complete with its own, unique microclimate. It is said that spending a few days down the mine (there are 140 beds available in the co-ed, underground dorm) guarantees good health for years. While you are here, you could play billiards or even visit the chapel which is sculpted in salt.