In a small pot combine the sugar, water, ¼ tsp. salt and cinnamon. Bring to a boil while stirring, until the sugar dissolves. Remove from the heat and add ¼ cup of the basil and lemon juice. Allow to cool to room temperature, then strain the syrup through a fine mesh sieve.
Place the watermelon, blueberries and strawberries into a large bowl then pour the cooled syrup over the fruit, tossing lightly to coat. Refrigerate and allow the fruit to macerate for at least an hour, stirring occasionally.
Whip the heavy cream with the powdered sugar and a pinch of salt until you have soft peaks. Serve fruit and syrup in bowls, topped with whipped cream and freshly torn mint and remaining basil.