(Optional) Rim each glass by dipping the edge in water or lime juice, then into cinnamon sugar.
Fill a cocktail shaker (or glass) with ice cubes.
Add grape juice, cranberry juice, lime juice, grenadine, and simple syrup.
Shake or stir well for 15–20 seconds.
Strain into serving glasses filled with fresh ice.
Top with lemon-lime soda or club soda for fizz.
Garnish with gummy worms, maraschino cherries, or blackberries.
For a spooky twist, swirl in heavy cream or drop in a scoop of vanilla ice cream.
Add edible glitter, mint, or spider decorations as desired.
Serve immediately and enjoy your Halloween party drink!