In Poland, most of the public museums have to set up one day with free entrance. Look below and check if you can find a free museum during your stay.
Note: these free days at museums can be quite crowded, and the free tickets are usually limited. I strongly recommend getting there in the mornings. Especially the World War II Museum is very popular – but totally worth it!!
[information below is actual for 08.10.2018. I’ll make my best to keep this up to date, but please check it before your visit]