For the dressing: In a small bowl, whisk together the lemon juice, olive oil, garlic powder, salt, and black pepper. (Alternatively, place in an airtight container and shake vigorously.)
In a large bowl, combine the avocado, tomatoes, and cucumber. Add the optional ingredients of red onions, cilantro, parsley, or dill weed for additional flavor.
Pour the dressing over the salad and toss to coat. Serve immediately.