There are actually two shops in the centre with reasonable film supplies (including medium format):
ul. Karmelicka 15
ul. Lobzowska 15
As for the "must see" places, I second the Wieliczka Salt Mine (it's just 15 km away from Cracow centre, reachable by 304 bus line). You just can't miss it.
I can also recommend an inexpensive way to see the city with a guide - google freewalkingtour (I'm not allowed to place direct links).
"krakow4u" website has many useful tips on what to see in the city and contains a lot of good pictures, if someone is looking for an inspiration .
If you want to see less obvious side of the city, google for "Krakus Mound" and "Liban Quarry", which is located right next to it.
Hope you'll have a great time!
Last but not least, if you want to drink a really good beer (not the regular one served in most of the pubs), visit either Omerta Pub in Kazimierz or House of Beer at Sw Tomasza 35.