Preheat oven to 400°F. Pierce potatoes with a fork; place on a baking sheet. Bake about 50 minutes or until tender.
Meanwhile, lightly coat a large nonstick skillet with cooking spray. Heat skillet over medium-low heat. Add olives and tomatoes to hot skillet; heat until warmed. Add onion; cook until tender. Add spinach, stirring until combined and heated through. Remove from heat. Stir in ½ cup of the feta cheese, the yogurt and salt.
Let potatoes cool for 10 minutes; cut a lengthwise slice off the top of each potato. Using a teaspoon, gently scoop out the potato flesh, leaving a shell about ¼ inch thick (each shell should weigh about 4 ounces). Divide spinach mixture among potato shells, spooning spinach mixture evenly into shells. Sprinkle with the remaining ¼ cup feta cheese. Bake about 10 minutes more or until heated through.