1. Combine sugar, salt, and 1/3 cup water in a small saucepan over medium-low heat. Simmer, stirring occasionally, 2 to 3 minutes or until sugar dissolves. Stir in cardamom pods and lime juice. Remove from heat; steep 30 minutes.
2. Pour mixture into a glass or plastic container; cover and chill 8 hours or overnight. Strain cardamom pods from syrup, and discard pods.
3. Arrange watermelon and mango on a serving platter. Sprinkle with mint. Drizzle with about half of syrup, and serve with remaining syrup.