1. Place chicken in a resealable plastic bag set in a shallow dish. For marinade, in a small bowl, combine lemon juice, oil, oregano, garlic, and pepper. Pour over chicken. Seal bag; turn to coat chicken. Marinate in the refrigerator for 4 to 24 hours, turning bag occasionally.
2. Meanwhile, in a medium bowl, toss together cucumbers, tomatoes, and red onion.
3. Drain chicken, discarding marinade. Place chicken on the rack of an uncovered grill directly over medium coals. Grill for 12 to 15 minutes or until tender and no longer pink (170 degrees F), turning once.
4. Transfer chicken to a cutting board; cut into bite-size pieces. Toss with cucumber mixture. If desired, serve on salad greens. Drizzle salad dressing over. Sprinkle with feta cheese and olives.