Slunce ve skle - Sun in the glass beer festival (Prague)

Hello,

I will be visiting Prague on September but only for three days, so I would like to know if it's worthwhile to go to the "Slunce ve skle" beer festival taking place in Pilsen on Sep, 19th. Is anyone here going there?
Also, is there any hidden gem or unmissable place that I should go to that is not so touristic? That applies for both sightseeing, places to visit as well as nightlife spots.

Regards,
Flavia.