Krakow is a very safe city, and it is rare to see any problems relating to crime. But one thing you do see a lot of are the girls holding umbrellas that try to get guys into strip clubs.
They wait near the main square (especially near Floriańska Street), and you can spot them easily. They are easy to avoid (a simple "no" and they'll leave you alone).
It is rare to be offered drugs in Kraków.
Crime happens in any city, but almost everyone who visits Krakow agrees that it is a very safe city. You obviously have to be careful of pickpockets, or angry drunks, but generally it is safe.
Read about exploring the abandoned Liban Quarry (and ex Nazi labour work camp)