In a large bowl combine tomatoes, cucumber, sweet pepper, red onion, thyme, and oregano. For dressing, in a small bowl whisk together balsamic vinegar and olive oil. Pour dressing over vegetable mixture. Toss gently to coat.
If desired, line a serving bowl with lettuce; spoon in vegetable mixture. Sprinkle with feta cheese. Makes 8 (1/2-cup) servings.