My ex and I enjoyed Cal Pep's for low-brow tapas. It's extremely tiny and tightly packed - people were lined up for stools 3 minutes after the doors opened for lunch...but well worth the wait!
I love Cal Pep but I hate the wait. I'm usually with friends from BCN and when they see the line they roll their eyes and suggest we go somewhere else.
One reason to go to Cal Pep is for their special dessert. It's olive oil on a slice of french bread with a square of chocolate on top. Then a little salt sprinkled over the whole thing. It sounds weird but the tastes all work together.
Not far from Cal Pep is a place called Sandwich and Friends. Walk by and look in the window. The big deal is that the decor has giant illustrations by Jordi Labanda. You'll see his style copied all over the place, but he was first to do it.
http://www.sandwichandfriends.com/