I've been to El Zocalo quite a few times and the only downsides are the prices are a little steep and the sangria is very disappointing (bottled). But the patio is famous for being awesome, the margaritas are delicious, I think their speciatly (I like the pama marg).
You've got to try the shrimp appetizer with the chipotle sauce, get a whole order or you'll be sorry you didn't. The soup that's on the menu is really good, I forget what it's called but it's perfect for lunch especially if you don't want to take a nap on your desk afterward. It's got potatoes and chicken, maybe some cilantro, it's yummy and light.
Also, Friday and Saturday nights they have live music and I believe they are open pretty late, midnight or one maybe?