Dark leafy greens include things like kale, collards, spinach, swiss chard, mustard greens, broccoli, and cabbage. Our modern diets usually don't include much of these, but here are some ways to add them back in:
1) add them to omelets or other egg dishes, along with other favorite veggies
2) add them (cooked or frozen to eliminate food poisoning) to smoothies
3) add liberal amounts to stir fry the last few minutes. Cooking shrinks greens considerably.
4) pile greens and beans on tortilla chips and top with cheese for an easy lunch
5) fill two whole wheat tortillas with greens and cheese and grill