Some of the obvious places to see:
1) Market Square, Mariacki Church & Sukiennice (Cloth Hall) - Krakow's heart, also happens to be the largest medieval city square in the world. History, souvenirs, cafes, restaurants and pigeons: all your heart desires witin a walking distance.
2) Wieliczka Salt Mine - officially on the UNESCO's World Heritage list. An underground salt mine that's been operational for almost 1000 years. All the rooms and halls inside have been sculpted out of pure salt! Phenomenal acoustics!!
3) Oswiecim-Brzezinka (Aushwitz-Birkenau) Concentration Camp - not for the faint of heart, but an important place to go if you want to experience some history.
4) Wawel Hill & Castle - the home of Polish Kings and Queens. So much to see, so little time. There is the Royal Chambers and the Armory, and the Treasury, and the Dragon's Den, and the Virtual Wawel Tour... See it all!
Quick Tips:
Off The Beaten Track:
Go to Wierzynek Restaurant on the Market Square. You will dine in the oldest and one of the finest quality restaurants in Poland, yet end up paying what you would at a cheap restaurant in the US or Europe.
Mariacki Church Tower - when you go visit the Mariacki Church talk to security / curator, and ask him if you could go up the tower and see the famous firefighter play the 'Hejnal' on his trumpet. If you play your cards right, for a little 'incentive' he'll let you climb the tower.
Best Way To Get Around:
Krakow is incredibly compact. To discover the old town walk everywhere. As a good introduction to the city you could take the horse-carriage ride which should only set you back about 60 PLN (). To go to Wieliczka or Aushwitz, take a bus tour (there are tours available in English and German), or get there yourself by train or bus (however, you'll be better off knowing some Polish if you choose that route). Local transportation is cheap and very reliable, but most of the schedules and maps are only in Polish. If you're going to take a taxi, ask the hotel to call one for you, as you'll be assured to get an honest cabbie and a discount rate.