Krakow is probably the most touristic city of Poland. With a well kept old town, a castle with mythical caves underneath, plenty of cultural events, cheap bars and also its proximity to Auschwitz it draws a lot of people from all over the world. The historic center is most of the time the first stop for everybody who visits.
If you feel like exploring more of Krakow, you just have to cross the Vistula river. In the south larger areas have been newly redone and modern architecture and interesting street art take over the city.