At the oddly shaped corner where Charleston and Fremont Street become Boulder Highway, in one of the older and more dangerous parts of east Vegas, you can find a great taco 24 hours a day at Pepe's! For 7 or 8 dollars you'll get a huge pile of authentic Mexican food. I'm partial to the Super Burrito with Carne Asada, or the Huevos Rancheros - yes they serve breakfast all day! Que gusta! Very authentic joint. Makes a great contrast to the Strip casinos if you want to cruise down Charleston for a break from the contrived slot palaces of MGM and Harrahs.
You might even see a real live mugging - just be careful it isn't you!