Combine sugar and water in a small saucepan; bring to a boil and boil 5 minutes. Remove from heat; cool slightly.
Place mint leaves in a small bowl; add sugar syrup, orange juice, lemon juice, and grated orange peel. Cover and let steep for 1 hour. Strain into a 1-quart container; cover and keep refrigerated.
To serve, mix 1 part lemon mint mixture with 2 parts water. Serve over ice and garnish with lemon or orange slices and sprigs of mint if desired.
Makes about 1 quart syrup, or 3 quarts of lemonade.