Bland and healthy foods to use as filler?

Human food. I want to get more out of burgers, meatloafs, omelets, burritos, sandwiches, but want healthier and cheaper fillers to stretch the quantity of food I yield.

i.e. At Chipotle, I add even more brown rice and beans at home than originally given. I also add a few eggs, whole wheat tortilla, whole wheat bread slices, grilled chicken, and tofu. I let Chipotle create the main flavor profiles, then I add easy fillers at home to create a bigger burrito bowl