Wieliczka is a small town just southeast of Krakow. There are assorted services that run from Krakow directly to the salt mine there but we found it much easier to just take the train from Krakow's glowny station. It's cheap, it's fast and it drops you off right in the center of Wieliczka from whence it's about a 10 minute walk to the salt mine.
The Wieliczka salt mine is the primary attraction in town. It's an enormous complex that has been expanding since the 12th century. The visitor center seemed pretty chaotic to us. Tickets for foreign language tours (like English) are more expensive than those in Polish and there's an additional fee for photography. Overall it's actually quite expensive (circa $35 US per person).
This map shows the route of the basic tour.
The tour starts out promisingly, descending a tight spiral wooden staircase into the mine. It takes quite a while to get down to 150m (~ 460 ft) below the surface. From here on it becomes quite clear though that the tour isn't so much about the mining and salt as it is about sculptures of salt. The sculptures are impressive but much of the tour becomes a guided walk between scenes.
The focal point of the tour is the salt cathedral. Everything in here is carved out of the salt (except for those giant wooden supports on the left side). Even the chandeliers are made from salt crystals.
Naturally, being a salt cathedral there's a salt altar, a salt nativity, a salt last supper and even a salt pope. The history that is volunteered on the tour is mostly limited to a list of famous people who've visited the mine over the years. Certainly though Krakow owes some of its medieval prominence to the wealth generated from this mine. There are a couple rooms of interest as far as early mining techniques but really most of the attraction is the mine itself.
We found the vast number of side tunnels, nearly endless series of ramps and stairs and scaffolding disappearing into darkness to be the most fascinating part of the visit. Long corridors like this one have names similar to city streets to help navigate the 300km (180 miles) of tunnels.
The town of Wieliczka probably wouldn't draw many visitors without the mine, but once you're here it's worth a little exploring. There is a lot of green space through the center of town and a few restaurants around the central square. Taking the train back to Krakow is a bit more difficult since they only leave every couple hours or so. Since we were in between times we took a shuttle bus instead. Most of these seem to just pick a random endpoint in the vicinity of central Krakow so we'd recommend asking where exactly they go. In our case we were fairly lucky and it left us only a short walk to our hotel.