I did two fantastic walking tours with http://freewalkingtour.com/ I strongly recommend doing their tours. You just enjoy and then tip as you see fit at the end. Great tour guides. I did both the Kazmierz tour and the Old Town Krakow tour.
A view of the central square, the largest in Europe.
Krakow is one of my very favorite cities in the world thus far. Beautiful 17th century architecture, clean, parks to run in, cheap, and great bars.
I had a great time staying at Greg and Tom's Junior Hostel. A bed was about $18, and with it came FREE DINNER every night. Amazing. Not only that, but in conjunction with Krakow City Tours, they had a nightly pub crawl, which for a grand total of $9 (30 zloty), you were led to a pub and served 4 or 5 shots of different Polish vodkas, then walked to a bar where you got pretty much all the beer you could drink, then staggered over to a club where your entry was paid. I went three different nights. Krakow provides more fun per dollar than any city in Europe Ive visited.