Even if you've had a massive hangover from the previous night's pub crawls, you'll sober up immediately when you go to Auschwitz-Birkenau.
The Auschwitz tour is a must-do activity when one is in Krakow, Poland. While the city with one of the largest student populations is rife with partying, it is a place steeped in history and this infamous site of genocide is a place where one can get a reality check.
Reading about the terrors the Nazis did is enough to shock but actually seeing the bombed ruins on the campsite, the blocks where millions were gassed, the collection of belongings brought by the prisoners, the cramped wooden planks that functioned as 'beds' for the prisoners, the endless lines of barbed fences and much much more insanity in the hell on earth, all bring a sobering image of the evils and insanity committed by and endured by man in recent history.
Definitely an experience for all to remember.
There are many tours in the city that organise excursions to the camp but best ask your hotel/hostel for a reputable one. Don't forget to allow at least more than half a day gone for the trip. Bring some food along and go to the toilet before visiting the camp! Try not to drink too much water on the trip!