We had a wonderful weekend in Krakow. If you want to do Auschwitz and the salt mines in one day I think you'll need to do an organised tour. There seemed to be a few companies doing them when we were there, certainly the hotel had lots of details on them.
We just did Auschwitz, we got the local bus there and back. The bus station took a little finding, but once there the staff were really helpful, told us the right bay to go to, bought the tickets on the bus. There seemed to be far buses than were actually on the timetable. Going we caught a small minibus type thing that didn't stop many places and was quite quick, coming back we caught something that looked like a coach, was a lot slower and stopped for a while in a nearby town with lots of classic Russian/ Eastern European 60s tower blocks. If you are the kind of people to turn up, go with the flow and figure it will all turn out OK in the end, then local buses are fine. But they did seem a bit random to the untrained eye, and poss not great if you are not too confident.
If the Xmas market is on, it is lovely just to walk round, as is the indoor market. We ended up buying lots of chess sets for people as Xmas presents. The hot wine was lovely and more potent that it tasted.
The only downside of the whole weekend was walking through the stunning town square at 11pm and some drunk 50 something Brits shouting "go on Densie, get yer tits out for Krakow"
It's lovely, hope you have a great time