Carnation Cafe's Veggie Burger is indeed very good (and I am not a vegetarian or vegan by a long shot), just be aware that most, if not all locations share the same equipment as meat when preparing vegan and vegetarian dishes, anything fried at a location will share the same fryers and oil, so if a location has fried chicken or fish, the fries will have been fried in the same oil. I know for some this won't be an issue, but for some it will be, so definitely something to be aware of. Some locations can accommodate using different equipment, but it may take longer to prepare.
Racho del Zocalo's Cesar Salad is actually Vegan if you get it without cheese, as they don't use anchovies in their sauce.
Also, stop in a city hall o guest services and ask for a list of all vegan options in the parks. Sometimes the info is a little outdated, but usually decent.