Fill your cocktail shaker halfway with ice, then add the orange juice, lemon juice, pineapple juice, and grenadine. Secure the lid and shake well for about 15 seconds until the outside of the shaker feels cold.
Fill a tall glass with fresh ice and strain your shaken mixture into it, filling the glass about halfway.
Top up your glass with ginger ale or soda water, leaving a little space at the top. If using bitters, add them now.
Gently stir everything together with a spoon, being careful not to lose the fizz.
Garnish with a slice of orange and pineapple on the rim of the glass, and serve immediately while it's perfectly chilled and bubbly.