Preheat your oven to 350°F (175°C). Grease a 9x5-inch loaf pan or line it with parchment paper.
In a large bowl, combine the grated zucchini, granulated sugar, brown sugar, eggs, vegetable oil, and vanilla extract. Mix well until fully combined.
In another bowl, whisk together the flour, baking soda, baking powder, ground cinnamon, and salt.
Gradually add the dry ingredients to the wet ingredients, stirring until just combined. If desired, fold in the chopped nuts and raisins or chocolate chips.
In a small bowl, mix together the ¼ cup granulated sugar and 1 tablespoon ground cinnamon to create the cinnamon swirl mixture.
Pour half of the zucchini bread batter into the prepared loaf pan. Sprinkle half of the cinnamon swirl mixture over the batter. Pour the remaining batter on top and finish with the rest of the cinnamon swirl mixture. Use a knife to gently swirl the batter for a marbled effect.
Bake in the preheated oven for 50-60 minutes, or until a toothpick inserted into the center comes out clean.
Allow the bread to cool in the pan for about 10 minutes before transferring it to a wire rack to cool completely.