Transfer all of the trays to the freezer and freeze until solid, preferably overnight.
In a small saucepan over medium heat, combine the sugar and water. Cook, stirring occasionally, until the sugar is dissolved. Let cool.
In a pitcher, combine 1/2 cup of the sugar syrup with the lemon juice. Add 3 cups of cold water, then taste and add additional sugar syrup if desired. Chill until ready to serve.
To serve, place 2 watermelon cubes, 2 blueberry cubes and 1 coconut cube in each of 6 rocks glasses. Top the glasses with lemonade, then garnish with mint. Let sit for 10 or so minutes to allow the cubes to melt slightly and flavor the lemonade.
Nutrition information per serving: 150 calories; 20 calories from fat (13 percent of total calories); 2 g fat (2 g saturated, 0 g trans fats); 0 mg cholesterol; 40 g carbohydrate; 2 g fiber; 33 g sugar; 1 g protein; 10 mg sodium.