In a large saucepan over low heat, melt butter. Stir in marshmallows until melted. Remove from heat. Stir in cereal until well coated. With a buttered spatula, press into a greased 13x9-in. pan. Cool.
Beat cookie dough and flour until combined. On a lightly floured surface, roll out dough to 1/4-in. thickness. Using a 2-in. tombstone-shaped cookie cutter, cut out 16 tombstones from dough. Place 2 in. apart on ungreased baking sheets.
Along bottom edge of each cookie, insert 2 toothpicks halfway into dough. Bake at 350° until edges are golden brown, 8-10 minutes. Remove to wire racks to cool.
Place water and green food colouring in a bowl. Add coconut; cover and shake or toss to coat. Toast coconut in a shallow pan in a 350° oven for 5-10 minutes or cook in a skillet over low heat until golden brown, stirring occasionally. Using black paste food colouring, tint frosting grey. Frost sugar cookies; decorate with brown decorating icing.
Cut cereal bars into 3x2-in. rectangles; spread with melted chocolate chips. Using toothpicks, insert cookies into cereal bars. Decorate with coconut and candies as desired.