How to Make It
Preheat oven to 350°F. Coat a 10-cup Bundt pan with cooking spray. Whisk together flour, brown sugar, baking soda, cinnamon, 1 cup of the granulated sugar, and 1/2 teaspoon of the salt in a large bowl. Whisk in eggs, oil, and zest until just combined. Drain mandarin oranges, reserving 2 tablespoons liquid from can. Fold mandarin oranges, pineapple tidbits, and reserved orange liquid into batter until just combined. Spoon batter into prepared Bundt pan.
Bake in preheated oven until a wooden pick inserted in center comes out clean, 50 to 55 minutes. Transfer cake in pan to a wire rack.
Bring butter, rum, water, grenadine, and remaining 1/2 cup granulated sugar and 1/4 teaspoon salt to a boil in a medium saucepan over medium-high. Reduce heat to medium; simmer, stirring occasionally, until slightly thickened, 5 to 8 minutes.
Poke holes all over cake in pan using a long skewer. Pour 1/2 cup rum syrup all over cake, letting syrup sink into cake and holes. Cover cake with plastic wrap; let stand at room temperature 4 hours. Loosen edges of cake from pan using an offset spatula; invert cake onto a serving platter. Brush with remaining rum syrup. If desired, garnish with cherries and orange peel twists.