The base is chocolate frosting coated in crushed oreo cookie "earth"; the vegetables are simply starburst candies molded into shapes, with green frosting dipped cornflake "leaves" and green M&M "peas." It was all surprisingly quick and easy to pull together, and the kids (36 of them in both my children's classes) loved them.
Credit to "Hello Cupcake" by Karen Tack and Alan Richardson for the idea.