Every undergraduate college at UCSD features its own food outlet with a small convenience food store. Muir has the newly remodelled Sierra Summit modeled after a ski lodge - sort of. It's probably your fastest bet for lunch and has a big selection including personal pizzas, make your own stir-fry, a grill, soups, you-name-it. The Ratskellar is located below Sierra Summit. It was closed all last year, but we assume is re-opening this year. It has your typical grill stuff and is open later than Sierra Summit.
Also close by are Revelle's Deli (South) and La Casa (North). Oceanview Terrace (North West) is a little newer. Oceanview's outside seating has the best view when you need to impress a visitor, and they have a pretty good salad bar. OVT is over on the way to RIMAC, it's kind of nice if you're coming back after a workout.