I often order Carnitas, and a side of pico de gallo, because the calories are simple to count (if you're pretty good at estimating portion sizse). Roasted chunks of pork, sometimes with cooked onion and/or bell pepper.
Usually there are also fairly simple grilled fish and steak dishes that are also easy to count (because they're not in mystery sauces). Fajitas can be very good, or they can be very oily. Shrimp Fajitas tend to be less oily (in my experience).
If I'm going to eat any bread, I always order corn tortillas, because they're usually about 50 calories each, where the flour tortillas are generally over 100 (and I now I have another reason to choose corn, as I may be allergic to wheat).
There's also a shrimp cockail (Coctel de cameron) I order if it's on the menu. It's often listed with the appetizers, but I order it as a meal. It's usually served in clear glass sundae glasses and/or brandy snifters, and often comes in several sizes, and you have to ask the server how big they are, because some restaurants have reasonable portions and the large is meal-sized and some have ridiculously huge portions, and the large may come out in a brandy snifter the size of a small aquarium.
To giver you an idea of what it's like I posted a link to an online version, and then my own recipe.
In a restuarant they usually serve it with saltines, but I don't eat them, and sometimes I'll ask for it without the avocado, but usually I just eat some of the avocado and leave some behind (the sauce the shrimp is in, is usually like gazpacho, a chunky cold soup). I'm not usually a gazpacho fan, but this is awesome.
The amounts of ingredients aren't really that important.
about 1/4 to 1/2 cup of
chopped green onion, small handful
chopped bell pepper and/or cucumber
chopped tomato (seeds and soft center removed - just the firm parts)
salsa of your choice, about 2/3 to 1 cup
(Ideally I like to use a fresh salsa or pico de gallo, but in a pinch I'll use Pace. Often fresh salsa eliminates the need for the onion, tomato, and cilantro).
finely chopped cilantro, couple tablespoons
avocado diced in small cubes (optional, If I use it, I only use a little bit of avocado)
cooked shrimp, about a cup
tomato juice (to thin the salsa to your taste - optional)
little squeeze of lime (I don't use if I don't have it)
pinch of sweetener if the salsa is too tart (optional)
Just mix everything together (sometimes I don't thaw the shrimp, I just put them in the salsa the night before and let them thaw and marinate in the salsa - but don't add the fresh ingredients till right before I'm ready to eat it)