Kraków is a bit city and at times it can seem that you are never more than 2 streets away from a general convenience/grocery shop.
A huge supermarket can be found on the top level of the Galeria Krakowska shopping mall.
There are also shops open 24/7 that sell alcohol and basic food (like candy/sweets, crisps/chips (often "Lays" brand), long life milk, soft drinks like Coca Cola, Fanta etc) and cigarettes. You can find these 24/7 alcohol shops throughout the city.
You can spot them by their name 'Alcohole'. They sometimes looked closed or have lots of wire mesh on the windows, but they will be open. Do not expect the staff to speak much english though in the 24/7 alcohol shops.
Prices are generally quite cheap in normal grocery and convenience stores, but 24/7 alcohol shops can be slightly more expensive.
Read about exploring the abandoned Liban Quarry (and ex Nazi labour work camp)