Located right on Plac Centralny, as it has been since Nowa Huta was constructed in the early 1950s, this has to be one of the most genuine milk bars in Poland. Not long ago it was decorated with simply fantastic rainbow motifs, but apparently that style went out of vogue and its back to being drab and colourless (those light fixtures are still pretty awesome though). Queue up and see if you can successfully order soup, entree and a drink for 10zł or less; it's very possible. Their opening hours are absolutely impossible to keep updated, so please check their FB page.
