Spray or brush a nonstick skillet with olive oil; set over high heat. On a work surface, sprinkle each of 4 tortillas with 1/4 cup cheese; top with desired fillings. Sprinkle with 1/4 cup cheese; top with 4 remaining tortillas. Grill until bottom tortilla is brown and cheese is melted. Flip; cook until brown, cheese is melted, and filling is hot. Transfer to a work surface; cut into wedges. Serve immediately with salsa and sour cream on the side.