This is a brilliant museum and despite visiting in August it was virtually empty ! Seems as though ,as it is situated about 15 minutes walk from the main square, lots of tourists appear to give it a miss. My advice - don't - go there - it is one of the best museums I have ever been in and was one of my (many) highlights of my visit to Krakow.
There are three floors of permanenant exibits plus other special exhibitions which you have to pay an additional entrance fee to see.
You could easily spend a whole day here to fully appreciate what is here. If you have a spare few hours then get along - even the kids will enjoy trying on the old military uniforms and the helmets. This 'kid' in his forties certainly did !
Add this museum to your 'must do' list when visiting Krakow.