Soak the apple slices in the lemon juice and water mixture for 5 minutes to prevent browning. Drain and pat dry with paper towels.
Pour the caramel sauce into two small bowls. Pour the melted chocolate into another bowl.
Fill the remaining small bowls with crushed pretzels, mini chocolate chips, Halloween sprinkles and chopped nuts if using.
Arrange the bowls at intervals across the board, leaving space between them.
Fan the apple slices in neat overlapping rows around and between the bowls, filling the board generously.
Place small spoons or toothpicks beside each dipping bowl so guests can drizzle toppings over their slices.
Cover loosely with cling film until guests arrive, then remove the film and serve.