There's this place called Andy's (I think) in the Heights. Google it. Hard to notice and might appear kind of dumpy, but decent and cheap food. And they serve breakfast all the time.
There's a taco stand on Huffmeister(by highway 6/1960), by Cy-falls high-school, the name of it evades me (I just call it "The Taco Stand) but it is amazing, and you wouldn't regret hitting it up if you're here on the NW. Try their barbacoa.