Hi Tiggster,
My wife and I spent a week in Krakow last year. We visited Auschwitz and the salt mines but you can't do them both in a day which is my reading of your question. There are several tour companies in Krakow who offer various trips and the Auschwitz day trip cost us about �15 each. Auschwitz - or Oswiecim - is about 40 miles west of Krakow and the tour is very organised due to the number of visitors and coaches at Auschwitz. Generally, you spend the morning touring Auschwitz I which was the original concentration camp and HQ for the entire Auschwitz complex of 50 camps. You then spend the afternoon touring Birkenau or Auschwitz II which was the extermiantion camp nearby. The organised tour is quite rushed as there is so much to see so you may be better hiring a car and wandering round yourself. Entry to the camps is free.
The salt mines are quite amazing and you can buy tickets from any tour company or hotel. It costs about �10 each to visit but the mines are very popular so try to visit during the week or first thing in the morning if you want to avoid the crowds.
There are many other things to do in Krakow too and it's a wonderful place to visit. We visited Oskar Schindler's factory and you can sit at his desk to sign the visitor book. We also walked several miles through Wolski Wood to the zoo which is the only one in the world surrounded by forest. There are long walks along the river as well as through Krajobrazowy Park. Kosciuszko's Mound and military museum are also worth a visit and they are near the zoo. There's also the castle and cathedral, although they are very busy.
Have fun. Poland is very cheap, the food is terrific and you'll love it as much as we did.