Lime syrup adds tang to watermelon and cantaloupe.
In a small saucepan, combine sugar and 1/2 cup water; place over medium-high heat. Bring just to a boil, reduce heat to a simmer, and cook until syrup has thickened slightly, about 15 minutes. Remove from heat, stir in lime juice, and chill in an ice-water bath.
Meanwhile, cut the melons into attractive 1/4-inch-thick slices. Transfer to a serving dish, drizzle with the cooled syrup, garnish with mint leaves, and serve immediately.