Well, I would definitely agree with John from Ontario that Krakow would be number one for the visitor with limited time. It is a jewel, and as he said, the history knocks you over. It was damaged by WWII less than the rest of the country, which was basically bombed and burned flat. I would definitely recommend going to Auschwitz,,,yes, it is traumatic, but I think we all need to see it and walk the ground. And I agree on the salt mine...sounds like a strange place to visit, but is a guaranteed wonder. However, if I had to choose, I'd spend time on the Rynek in Krakow, tour Wawel, wander the back streets in Old Town, and go to Auschwitz. This will also be a good choice for you because it is closest to Prague and Budapest which you said you're also visiting.
Have a great trip. Corliss KC