Even though it is easy to go to the salt mine under your own steam I would advise you to consider going on a group excursion. There are a lot of advantages.
The visit of the mine is in a group anyway besides English language tours are strictly timetabled, 6 per day in summer and 2 in winter.
The queues at the ticket office are horrendous, especially in July and August.
An excursion from Krakau is zl 90, this includes transfer and ticket. (If you do it yourself: entry zl 36, retour onthe train zl 5)
The Wieliczka salt mine contains an entire town below ground with sanatorium, theatre, church and café! Everything from stairs to chandeliers is made from salt.
The lowest level is 135 m down. here is the five-star attraction; a chapel completely carved from salt, incuding the altar, a chandelier just everything. After this the other levels seem ordinary.
The website has a good selection of photos and gives an excellent impression of the mine
