Prepare and bake the cake mix according to package directions for 9-inch cake pans. Cool in pans on wire racks for 10 minutes. Remove from pans and cool completely.
To assemble, place one cake layer on a serving plate or tray. Spread about 1/3 cup white frosting over layer. Place the second cake layer on a cutting board with the rounded side up. Using a long serrated knife, start at the center of the cake and cut at an angle toward the edge of the cake to remove a diagonal piece of the cake layer (this will create the "hill"). Lightly dust off crumbs with a pastry brush then place the layer, cut side up, atop the cake on serving plate.
Frost top and sides of cake layers with some of the remaining white frosting (if frosting clings to crumbs, freeze cake for 30 minutes before frosting; any crumbs that do cling to frosting will be covered with coconut). Lightly press coconut into top and sides of cake. Set about 2 tablespoons white frosting aside. Beat powdered sugar into remaining frosting to get a stiffer consistency, and tint frosting green. Spoon green frosting in to a pastry bag fitted with a leaf tip.
For trees, use a serrated knife to cut pretzel rods into varying lengths. Using green frosting, pipe leaves onto uncut ends of pretzel rods. (You should make 5 or 6 total trees.) Insert pretzel rods, cut sides down, into one side of the cake.
Cut fruit leather into 6-inch pieces; roll one end toward the center to resemble a sled (make about 4 sleds). Arrange sleds and cookies on the sloped part of cake so it looks like they're sledding. Use the reserved white frosting to attach the cookies to the sleds.
Dust trees with powdered sugar so it looks like they've been dusted with snow. Makes 12 to 16 servings.