Place the sugar, water, orange juice and orange zest into a small saucepan and stir to combine. Bring to a boil, then turn the heat to low and simmer for 15-20 to thicken slightly. Set aside to cool, then store in the fridge until cold. Mix together all the fruit in a large bowl and pour the syrup over the top, tossing gently. Garnish with mint leaves and serve.