Chill the glasses: Place 4 tall glasses in the freezer for 5–10 minutes so your soda stays extra cold. While the glasses chill, set up your ingredients and tools.
Dice the pineapple: Dice pineapple into small, even cubes (about 1/4–1/2 inch) so it releases juice easily and muddles quickly.
Muddle the base: In a pitcher, add pineapple chunks, fresh lime juice, sweetener, and a pinch of sea salt. Muddle firmly for 45–60 seconds until the fruit is very juicy and fragrant. Let it stand 2 minutes to macerate; while it rests, slice a few lime wheels for garnish.
Strain (optional): For a smoother sip, pour the mixture through a fine-mesh strainer back into the pitcher, pressing to extract as much juice as possible. Discard pulp (or save it for smoothies).
Stir in juice: Add the chilled pineapple juice to the pitcher and stir until the mixture looks bright and uniform.
Build the sodas: Fill each chilled glass to the top with ice. Pour the pineapple soda into each glass, then top with a splash of sparkling water or club soda for more fizz.
Garnish and serve: Tuck a mint sprig, a lime wheel, or pineapple spear into each glass and serve immediately.