I visit Krakow about five times a year. I love it, lovely people, hearty food, great jazz clubs, stunning architecture.
I recommend Kazimierz too, the old Synagogue is lovely and the whole area is quaint and unspoiled with some great pubs (Omerta if you like beer!) Just over the new cycle bridge is Podgorze where you can visit the Schindler factory- now a museum after a lengthy refit. You might recognise bits of Kazimierz from Spielberg's film.
I've been to Auschwitz a few times. It annoyed the hell out of me that schoolkids were walking round munching sandwiches and giggling, then I got annoyed at myself for letting it bother me. It's a chilling place, it's just too much to take in. The corridors are lined with photos of inmates with their date of incarceration and date of death. Very few lasted more than a year. The mounds of hair and glasses are still there. And the old huts are still standing, or at least the chimneys are, the wooden parts have rotted away.
Gorecki, the Polish composer heard about a message written on the cell wall of gestapo headquarters near Krakow by an 18 year old girl. He composed a symphony using the girl's words and rare permission was granted to film the symphony at Auschwitz, unbearably moving:
http://www.youtube.com/v/miLV0o4AhE4&rel=1It's a place hard to describe in words, Gorecki's music is the most chillingly beautiful thing I've heard, I hope it's not out of place to post it here.