I wasn't in Kraków for a long time. I remember when almost half of Main Market Square was closed because of archeological works. And when I came back I had an opportunity to go and see the Underground Museum. It is wonderful - interesting, entertaining and smart. You can really see how did it look the Centre of Krakow in the past. With some animations, movies and moquettes you can easily time travel.
Before going there check on the museum website promotional ticket prices.
I recommend it for everyone. Not to be missed!