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.