Chipotle is one of the best fast food chains for eating wild. They have higher quality meat and ingredients, and it's fully customizable. There aren't a lot of menu options, so here's how to do it.
1. Form: Order the salad, not the burrito - you can get any of the same ingredients, you just lose the tortilla
2. Filler: Pass on the rice and beans, but go for the sauteed vegetables - rice and beans are filler anyway, which is why they put them before the meat. Realize that Chipotle wants to lower their food costs by stuffing you with the cheap stuff.
3. Meat: Pick any meat - I love carnitas or barbacoa, but they're all good. *See bonus technique below for getting more meat without paying for it.
4. Toppings: Add pico de gallo and guacamole, maybe add sour cream, but pass on the corn and cheese - Sour cream isn't orthodox paleo, but it's full of fat, which is much better than anything sugary.
5. Dressing: Pass on the dressing, but use some of the hot sauce instead. Mystery dressings can be loaded with sugar and gluten. Just ask for hot sauce (medium or hot) on the side, and use it as your dressing.
* Bonus Technique: There are ways to get more meat without paying more. When you skip the rice and beans, your salad will look a bit scrawny. Don't ask for extra meat (they'll charge you), but say something like "Make sure you put enough barbacoa on there" or "Don't hold back" or "That salad looks a little empty, can you make sure you've added enough?" (Hat tip to Clark for the bonus technique.)
* Updated Bonus Technique: Commenters Sanjay and CJG suggest another brilliant trick to get more meat. Ask for half of two different kinds of meat. It's hard for them to do half spoonfuls, so the end result is that you get more. This is a highly advanced technique and should not be tried by novices.
And there you have it. You'll be able to fill up for under $10, you won't fall asleep afterwards, and you won't have to cheat on that late night run to go grab some chow with your buddies.