It’s tough to think of a type of advertising that’s been around longer than outdoor. Perhaps word of mouth?

Yet outdoor is hotter than ever thanks to creative ideas, technological innovations, and its ability to appeal to a captive audience.

Take McDonald’s, for instance.  The fast food chain has been doing some interesting things with billboards in Chicago, near its headquarters of Oak Brook, Illinois.

McDonald’s ad agency, Leo Burnett, hooked up with a horticulturist to create what it calls a “vertical garden” on a billboard across from Wrigley Field. The billboard uses sixteen types of lettuce, ranging from Romaine to Radicchio, that start as seedling and grow into full plants that spell out the words “Fresh Salads.” No word on its shelf life.

Here are a few other offbeat outdoor ads McDonald’s and Leo Burnett have cooked up:

• Took a snowplow — painted orange — for a spin through city during a heat wave to publicize the chain’s Arctic Orange Shakes.
• Put up a giant sundial billboard that featured breakfast options for every morning hour, like a cup of coffee for the early risers or a Big Mac for those who like to sleep in.
• Stretched a 12,062 sq. ft. banner across the bottom of a swimming pool — with the help of a couple of scuba divers — to market the 42-ounce Coke that McDonald’s was selling.
• Used a trolley — designed to look like 3-D ice with hand molded icicles — to distribute coffee coupons to potential customers, publicizing its iced coffee during the hot summer months.