Tex Mex is really easy and versatile to make.
Cook some meat with a little chili powder, garlic, and onion. Optional: green chiles, red pepper, jalapeno, etc. You can use beef, pork, chicken; it can be ground, chopped, pulled, etc.
Add some beans (black, pinto, refried, white, etc) and toppings (sour cream, cheese, salsa, lettuce, tomato). Use some form of tortilla (flour, corn, taco shell, chips) and you have a Tex Mex!
Burritos are a great portable food. Take a large flour tortilla (10 inches or larger) and stuff with your choice of meat, beans, toppings, and rice if desired. Wrap it up until the filling is completely encased.
Tacos are messier because the filling does tend to drop out. But with tacos, you can get away with using smaller tortillas and more filling.
Nachos are delicious, melt cheese on top of chips and then add toppings.
Quesadillas are also tasty and portable. Fill a tortilla with cheese and meat (beans optional), fold in half, and cook in a skillet until the cheese is melty and the tortilla is crisp on each side.
Oh, and don't forget guacamole, an excellent accompaniment to any Tex-Mex! It's made by combining mashed avocado, salsa (jarred or homemade), and a little lime juice.