The Wieliczka Salt Mine is one of the UNESCO World Heritage Sites.
It’s a mine, so you can visit typical “things” you would expect to find in a mine.
On the other hand, it’s like a museum, with many exhibitions made of salt, there is even a chapel.
It’s located nearby Cracow, so definitely visit the mine while being in Cracow.
For details, check the official website of the Wieliczka Salt Mine.
Cracow (in Polish: Kraków) is one of the most beautiful cities in Poland,
and I highly recommend you to go there!
Just be aware, that since some years, Cracow is a very popular tourist destination, it maybe overcrowded, so maybe it lost some of its charm (?).
Anyway, if you’re bored with Cracow, go to nearby Oświęcim (very importnat place, known for known for being the site of the Auschwitz Birkenau concentration camp) and Wieliczka Salt Mine;
or a bit farther – go to mountains.