Press mixed nuts into sides of cake. (If frosting has dried, brush sides of cake lightly with water to moisten.) Place coconut in a resealable plastic bag or a bowl. Add food coloring. Knead the bag to evenly tint the coconut (see how-to) or stir coconut in bowl until evenly tinted. Sprinkle coconut over cake. If desired, sprinkle blue colored sugar for a pond.
Arrange Graham Cracker Barn and Silo on top of cake. Arrange Sugar Cookie Farm Animals on top of cake. For fencing, crisscross the pretzel sticks at 1- to 2-inch intervals, then lay additional pretzel sticks across. Use shredded wheat biscuits to make hay stacks. Makes 20 servings.