In a small bowl, combine orange juice, vinegar, and sugar. Stir until blended and set aside.
Place cucumber, blueberries, strawberries, and apple in a large bowl. Top with the orange juice mixture and stir gently to coat.
Cover and refrigerate for at least 1 hour (overnight is best). Stir gently just before serving. Garnish with mint, if using. Enjoy!
PER SERVING (450g): 169 calories, 1g fat, 95mg sodium, 41.5g carbs, 7g fiber, 29.5g sugars, 2g protein