I can highly recommend Free Walking Tour Krakow. It amazing tour, you can learn about Krakow's history, and see the most important site and monuments. Tour guides are very informative insightful, entertaining, and funny.Information about this tour you can find on http://freewalkingtour.com almost 7 years ago
Wavel Castle & Auschwitz are must sees. Kasimierz is a really cool place for nightlife. Krakow is absolutely incredible to walk around.
One of the most enjoyable things was the "Communism Tours" of the nearby Stalinist Showcase city of Nowa Huta with Crazy Guides: http://crazyguides.com/ This is one of the best tours I've ever done. Informative, hilarious, insane. They drive you around in an old East German Trabant.
Although I didn't see it, the Home Army museum is also in Krakow: http://www.muzeum-ak.krakow.pl/english/index.php about 6 years ago