Cut 2 eggplants, 2 zucchini, and 1 large red onion into 1/2-inch slices. Cut 2 tomatoes in half, and shuck 6 ears of corn. Brush the vegetables with olive oil and sprinkle them with salt and pepper. Place everything on a grill over medium heat and cook, turning frequently, for 10 to 15 minutes or until fork tender. Serve hot, reserving one-third of the cut-up vegetables and 2 ears of corn for another use.