For anyone who has to occasionally eat on the road, I just went by Taco Bell and ordered their chicken Cantina Tacos. It's corn tortilla's onion, cilantro, and lime. That's it. Super tasty and low call too (www.tacobell.com has nutritional info but I can say it's better than pretty much any other fast food option). Right now these are limited time, but I hope Taco Bell will decide to keep making them.
Also, most Mexican restaurants have these types of tacos, labeled Tacos a la Parilla or something. I first had this type of Taco at a Fiesta that is held in Raleigh every year, called Fiesta Del Pueblo. It's a huge multi-Hispanic cultural festival, and if you are in the area, you should certainly go. But I digress.
Anyway, these are super tasty tacos, filled with your choice of meat, onion, lime and cilantro. All the flavors go together so well, you really don't miss cheese, guacamole and sour cream.