While not in Krakow itself, Wieliczka Salt Mine is easily accessible from Krakow by train or bus, and is a worthwhile day out!
The town of Wieliczka is about 45 minutes from Krakow Glowny, and once in town, it's about 15 minutes walk to the mine entrance.
There are several guided tours daily (in English or Polish) that will take you into the heart of the mine.
The first stage is a descent down many, many levels of stairs, after which you pass through ever-more awesome rooms and chambers.
The true beauty of the mine is the detail within each of these chambers - from statues and religious symbols to the Grand Chapel, EVERYTHING has been hand-carved over many years.
Obviously, the highlight is the Chapel itself, a massive open chamber with huge chandeliers that has served as a functioning church.
Happily, you won't have to walk back up and out, there is a lift after the restaurant and gift shop at the end of the tour.
If I had to sum up the mine in a word, it'd truly be "Awe-inspiring".