For guacamole, seed and peel avocado. In a mixing bowl coarsely mash avocado. Add the medium tomato, onion, the 1 tablespoon lemon juice, and salt. Cover the surface of the guacamole with plastic wrap and chill up to 4 hours.
For marinade, in a shallow nonmetallic dish combine oil, cilantro or parsley, lemon peel, the 2 tablespoons lemon juice, chili powder, cumin, and pepper. Add chicken to marinade, turning to coat. Cover and chill about 1 hour.
Drain chicken, reserving marinade. Grill chicken on an uncovered grill directly over medium coals for 5 minutes. Turn chicken and brush with marinade; grill for 7 to 10 minutes more or until chicken is tender and no longer pink. (Or, place chicken on the unheated rack of a broiler pan. Broil 5 to 6 inches from the heat for 10 to 12 minutes, turning and brushing with marinade once.) Wrap flour tortillas in foil and heat on grill or in oven during the last 5 minutes of cooking chicken.
To serve, cut chicken into bite-sized strips. On each tortilla, arrange chicken strips, shredded lettuce, shredded cheese, chopped tomato, and sliced olives. Roll up tortillas, tucking in sides. Serve with Guacamole. Makes 4 servings.