Slice 2 small zucchini and 2 small yellow squash 3/4 inch thick. Thread 2 pounds boneless, skinless chicken thighs, cut into pieces, on 8 metal skewers, alternating with zucchini and squash.
Whisk together 1/3 cup spicy-brown mustard, 3 tablespoons dark-brown sugar, 3 tablespoons olive oil and 1 teaspoon garlic powder in small bowl. Brush over skewers.
Grill or broil for 10 to 12 minutes, turning halfway through cooking, until chicken is cooked through. Serve over rice. Makes 8 servings.