The Rynek Underground Museum in Krakow is a neat little museum underground in the main square. It can be a little bit hard to find (its entrance is in the Cloth hall building – on the wall facing the St. Marys Church).
The Rynek Underground museum is a new museum (opened in late 2010) and has some high tech exhibitions (with lots of holograms) to show the atmosphere of Krakow 700 years ago. There are many interactive screens and lots to play with.
It costs 19zl to get in (pensioners/students 16zl).