Start with a handful of cooked flavored "Mexican" rice (I don't know how they make it, but our recipe is not bad, if you have the time for it). Throw in a ladle-full of cooked pinto or black beans. Then, a dozen of good corn chips/crisps, hopefully unsalted; some chopped lettuce and red cabbage; some shredded white cheese (Mexican manchego?); salsa of your choice (green spicy salsa is my poison at the cart), and some ranch dressing. Unless you are averse to it, cilantro/coriander leaves make it even better.
This is truly "fast food", since they make it in front of you in under a minute, including pauses for questions ("pinto or black?", "which salsa?", "ranch?") and yet it is delicious. I happen to like my chips to be soggy in this salad, so I let it sit for about 15 minutes, if I can.