Place 1 cup of the water and sugar in a small pot. Set over medium-high heat and bring to a boil. Boil, stirring, until the sugar has completely dissolved and the liquid is clear, about 1 minute. Mix in the sliced nectarines and heat through 2 minutes. Remove from heat and puree the mixture in blender or food processor, or in the pot an immersion (hand) blender. Cool the mixture to room temperature.
When cooled, pour the nectarine mixture into a serving jug. Pour and mix in the remaining 1 1/2 cups cold water and the lemon juice. Wrap and refrigerate until needed. Nectarine-flavoured lemonade can be made the day before needed.
When ready to serve, fill glasses half full with ice cubes. Drop 3 raspberries or blackberries, and 3 blueberries into each glass. Pour in the lemonade. If desired, garnish each drink with a ½ lemon slice and a mint sprig.
Recipe Options: Instead of nectarines, peel and use two ripe peaches in the lemonade. Instead of berries, try pitted, fresh cherries as the garnish for each drink. To make an adult, alcoholic version of this lemonade, add a splash of vodka to each drink.