Try La Calle at 10th & Broadway for quick, high quality mexican food. I like the tacos and an aqua fresca, but they have burritos and other similar fare. You can eat in or take out. Same owners as Tamarindo, which is another great nearby restaurant for not-so-quick, sit-down Mexican small plates.

Le Cheval is also a perennial favorite for Vietnamese @ 10th & Clay - their quick service and abundant seating capacity make it a good place for a short, sit-down lunch. They have a sizable menu with lots of tasty options. I personally recommend the soft stirred fried noodles with curry, watercress salad with beef, and salt & pepper calamari as 3 of their best plates.

If you can eat off hours, avoid the lunch hour rush and go to Rattos a bit early or late on Washington between 8&9th. It's an Italian deli that makes great fresh sandwiches to order, with some indoor seating. But prepare for a 20 min takeout wait if you dare to go at noon!

Finally - Fountain Cafe in City Center has a great salad buffet if you are up for healthy (or not so healthy) takeout. Very fresh, more salad options than you could ever need, and priced very reasonably.

Le Cheval is reliable & quick if you want to sit down & like Vietnamese. For to-go food, Rattos Deli & Cafe 817 are great but assume a 10 min wait. If you need it REALLY fast Starbucks has great sandwiches to go.I recommend the chicken salad. Yum!

There's also City Cup on Jefferson St. between 12th & 13th at the bottom of the parking structure. The cafe has coffee, breakfast (for lunch), prepared salads and sandwiches, in addition to quiches, turnovers and a taco salad bar. I run here when I literally have 5-10 minutes to grab some food. Adjacent to it is City Grille, which has actual meals. The convenience to this place is that it is so close to the office. If you do have time to spare, then I would recommend the above suggestions.

Rio California is one of my favorites:
http://www.urbanspoon.com/r/6/89895/restaurant/Rio-California-Cafe-Oakland