In the town the old centre is fabulous - the square with the cloth market(?) is very impressive ... The Wawel (?) castle is very impressive also... the old Jewish area is worth walking around also
Further out you should definitely visit Auschwitz - not exactly an uplifting place to go and see but shouldn't be missed. The Salt Mines (can't remember the name) are supposed to be fabulous (I didn't get to them) - on the World Heritage list etc.
If you've extra time the High Tatra mountains to the south around Zakopane are very impressive also - skiing in winter and Lakes & Mountains type thing in the summer.