If you interested in having a taste of some local vibe while you in Kraków consider suggestions below.
This is kind of local speciality sometimes called "grandma of hamburger". Kind of a sandwich/burger with pulled pork, sauce and bunch of other stuff made with special recipe. It used to be prepared in Kraków for centuries as a way for serving affordable meal(and apparently for utilizing food left-overs).
You Can have it at Andrus food truck by św. Wawrzyńca street(50 meters away from Stara Zajezdnia) or at some restaurants(I tried it at Zalipianki Ewa Wachowicz at Szewska 24 next to Planty and Teatr Bagatela tram/bus stop at it was delicious).
Kiełbasa z Nyski
Which is grilled sausage served on street from Nyska(kind of truck that was popular in Poland during so called communist era) at evenings in one specific spot in Kraków(at Grzegórzecka 1 next to "Hala Targowa").
It is very popular food-truck spot to eat late at evenings monday-saturday from 8. pm till around 3. am. It gained popularity during the times when there was little other places to eat during night, and because of good quality and nostalgia(it is served in old school, very simple way). Usually it takes at least a few minutes waiting in line to get served.
Przypiecek - pierogi 24h
If you stay out reaally late - you don't have to neccesarily eat at some chain restaurant - you Can taste polish pierogi at the heart of Kraków at Sławkowska 32(near Stary Kleparz bus/tram stop). Decent quality and reasonable price. Pierogi is one of the most reknown polish dishes - kind of dumplings - usually consist of meat, cheese or some fruits wrapped by dough and cooked.
Probably the most popular snack in Kraków - sold on streets from carts. It's basically the ring-shaped piece of bread usually covered with sesame seeds, salt or poppy seeds. Good little snack while you walk around Kraków.
Jadłodajnia u Stasi.
If you want to eat whole typical lunch(or rather obiad) like locals - in the heart of Kraków you Can find Jadłodajnia u Stasi at Mikołajska 16. You have to enter the gate, and pass pizzeria and go through courtyard- then sit inside the small room and wait to be served. This place is known from serving good polish food for decades already. It's open monday-friday between 12 and 5 pm.
There are also milk bars - small and cheap restaurants with polish dishes(not neccesarily milk-based). In many of them you Can still sense the vibe of previous, communist era. Personally I was often eating at Górnik at the corner of Czysta and Dolnych Młynów, or Uniwersytecki, at Czysta 5.
Just a few minutes walk from Main Market Square there is actual market where you Can purchase fresh local groceries, and bunch of other stuff.
Feel free to add something, correct me or share experience! Smacznego!