Krakow is a fantastic place, I really loved my visits here, and the centre is very well preserved and oozes history. The main square and Wawel hill complex are highlights of central Europe and contain enough history to keep anyone entertained. The Jewish quarter is very interesting as well, but I must admit the though of what went on during the Nazi occupation of the city really hit home when I was walking around it on a very very cold winters night.
The main sights are easy to get between and they are supported by a network of underground bars which are fantastic, especially in the Jewish quarter, they all seem to be in caves a massive fire risk but really interesting and cheap none the less. I would also strongly recommend CW Bruwar's which sells beer by the metre!
Krakow is very interesting and a great base for exploring the nearby UNESCO sites; Auschwitz, Wieliczka and Kalwaria Zebrzydowska (although I did not get to the later the others are easily accessible from the central train station)
If you are in Central Europe then Krakow is a must!