With a long sharp knife, cut a thin slice from watermelon so melon lies flat. Using a small sharp knife, lightly score a horizontal cutting line around middle of the melon, leaving 3 in. on each side of stem end unscored for the windshield. Next, score a vertical line over top of melon, connecting both ends of the horizontal line.
Using the long knife, cut through melon along the vertical cutting line. (Stop cutting at the horizontal cutting line.) Then, cut through melon along the horizontal cutting line. Gently pull away top portion of car.
Cut a thin slice from the end of the top portion for the convertible top. Trim slice to fit on tail end of car; set aside. Remove fruit from both portions of melon; cut into balls or cubes and set aside. Attach convertible top with toothpicks.
For side mirrors, cut a 1-1/4-in. notch from each lower corner of windshield. Place a strawberry half in each notch; attach with toothpicks. For windshield, using a sharp razor blade or small knife, score a 3/4-in. frame around perimeter of windshield. Carefully remove watermelon rind within frame of windshield.
For wheels, attach two lime slices with toothpicks to each side of melon. Cut the remaining lime slice in half. Using a toothpick, place one portion centered below windshield for a mouth.
For eyebrows, remove fruit from the remaining half slice; cut peel into two pieces. For eyes, cut ends from red grape and cut grape crosswise in half. Using toothpicks, add grape halves to the marshmallow halves and position on windshield; attach eyebrows. If desired, add a mandarin orange smile.
In a large bowl, combine the green grapes and remaining mandarin oranges with reserved watermelon. Spoon into car.