1. Cut bell peppers into 1-inch-wide strips. Cut squash and zucchini lengthwise into 1/4-inch-thick slices. Cut onions into 1/2-inch-thick slices. Lightly coat vegetables evenly with cooking spray.
2. Grill vegetables, covered with grill lid, over medium-high heat (350° to 400°) 3 to 5 minutes on each side or until tender. Remove from grill, and sprinkle evenly with salt and pepper.