There are two major day trips you take from Krakow: the Wieliczka salt mine and Auschwitz. I'd planned to go to Auschwitz my first full day in Krakow, but then it was really rainy so I decided to put it off a day. (Yes, I recognize the irony of putting off a visit to Auschwitz because I didn't want to endure a little physical discomfort.) Anyway, the salt mine is underground, so I figured it was a good bet on a rainy day.
The Wieliczka salt mine was mined from the 12th century to the 1960's. It's huge and it's a UNESCO World Heritage Site, so it's got serious tourism cred. This is the only picture I have:
While the miners carved, they dug all sorts of rooms and chapels and carved sculptures out of salt and stuff. I didn't pay the extra $4 to take pictures inside because I figured it would be low light and not worth it...I kind of wish I had, because the one salt cathedral was wild and reasonably brightly lit, but whatever. You're going to have to imagine it. Also imagine: a bunch of salt statues of people who visited the mine, from Copernicus to Pope John Paul the 2nd, who is basically everywhere you look in Poland.
To get into the mine you walk down 378 wooden steps. I knew there was an elevator on the way out, and I figured it would be, you know, an elevator. No. It was an insanely fast, unlit double-decker metal cage. They crammed eight people into each compartment - I felt like eight skinny people would fit ok, but my group was not that skinny. And the doors opened inward, so we had to squeeze extra hard to get the doors open and shut. It was...exciting. I don't recommend it if you're claustrophobic.