Wolf Creek just added a tap room at their brewery. I've only tried one of their beers but they might be worth checking out if you have time. http://www.wolfcreekbrewingco.com/locations.html
Magic Mountain makes you go through airport like security. The last thing they need to do is feed beer to those animals.
Hey man, I live in Valencia. I heard they just put a sports bar in the park that supposedly serves beer. I would second the Newhall Refinery recommendation.