1. Place oven rack in middle of oven and preheat to 375ºF. Line 2 large baking sheets with parchment.
2. Combine confectioners' sugar, coconut, egg whites, vanilla and salt in a medium bowl and mix with a rubber spatula. Add green food coloring and stir in completely until the dough is desired color. Dough will be sticky.
3. Drop heaping tablespoonfuls of dough onto baking sheets. Moisten your fingers with water. Using your index finger, make a 1/2-inch hole in center of each cookie. Carefully flatten dough with your fingers into a 3-inch wreath shape. Remoisten fingertips with water as needed. (Cookies don't spread as they cook, so leave about 1 inch between them on the baking sheet.) Repeat with remaining dough. Press a few M&M's Minis or other decorations into each cookie.
4. Bake cookies until firm and light brown around edges, 8 to 10 minutes. (Cookies will firm as they cool; they are very soft coming out of oven.) Let cool on baking sheets on wire racks. Repeat with remaining dough and candies.