4th St Berkeley for lunch
I love Zut on Fourth. It's not casual, but it's definitely not fancy. We have taken our kids, and just walked in without reservations. Great burger, solid soups and salads and apps, amazing cocktails (corpse reviver!), and a very nice ambiance. I would say you can't go wrong there.
Second Tacubaya, too! I'd go for the seasonal specials - they always have a quesadilla, a tamal, and a tostada with very interesting seasonal ingredients like yam, butternut squash, pomegranate seeds, huitlacoche, etc. Not your standard Mexican by any means.