What type of place are you staying in? Stayed in a self-catered apartment and the staff were excellent in recommending places.
I ate somewhere with pierogi on the menu everyday! I can't remember any names sorry, as it was a few years ago.
A useful bit of advice that we were given was that if you're paying in cash, if you say 'thank you' when handing it over, it's interpreted as saying 'keep the change' as a tip. If you'd like to get your change and arrange the tip yourself don't say anything.
In addition to Aichwitiz/Salt Mines that some have said (you can do both in a coach tour that takes a day and gives you a meal), I'd say the Jewish Quarter, we didn't really get the time to explore there. If you can arrange a tour (in a golf cart type thing), you can also get really good history. Krakow is also filled with churches, some are really beautiful, St Mary's Basilica is one, to have a look inside.
I'd say the castle isn't really a must-do, if you need to choose between things.
Also, random recommendation, if you're an adventurous type that's into horror, a trip to Lost Souls Alley is unlike any experience I've ever had 😂 take that as a warning