NEW YORK Leo Burnett has created some eye-catching billboards for McDonald's in the past, most notably a breakfast promotion featuring a sundial that tracked the movement of the sun with the fast-food chain's "M" logo. The agency has followed up that award winner with a dynamic new effort outside Chicago's Wrigley Field that is equally compelling. To advertise McDonald's healthier fare, the Chicago agency created a "living billboard." It's a garden of sorts, spelling out "Fresh Salads" with 16 different types of lettuce. Now that's a fresh idea!