I spent one day in Krakow, one day in Plaszow, and five days at Oswiecim. The five days I spent at the Auschwitz Bireknau is unforgettable. We were a small group of 12 people for a summer academy for Holocaust studies, so everyday we had lectures, walking tours and workshops inside the Auschwitz museum. The experience is indescrible, walking through the railway junctions for selections and burnt down gas chambers was heartwrenching.
Krakow has many sites of the former Ghettos that can be tracked back to and the most memorable one is a town square with many empty chairs representing each person rounded up by the Nazis and sent to an extermination camp. From that camp no one came back. There was a pharmacy on the corner of the square where the owner, a pharmacist witnessed the whole process.
Krakow itself is not the memorial part of the trip because of the terrible background it provides. To me it's a backdrop of a horrible crime against humanity, and the path towards genocide opened from there.