Combine 1 cup granulated sugar and 1 cup water in a small saucepan. Stir to combine and bring to a gentle simmer over medium heat. Cook until sugar is fully dissolved, about 3–4 minutes. Do not boil vigorously; you only need a clear syrup. Remove from heat.

Allow the syrup to cool for 10–15 minutes so it is warm rather than hot before adding delicate flavorings.

Flavoring

Add concentrated flavors

When the syrup is warm (not hot), stir in 2 tablespoons fresh lemon or lime juice, 2 tablespoons fresh orange juice, and 1 teaspoon vanilla extract. If using, add 1 teaspoon grated fresh ginger and/or 1 teaspoon citrus zest for extra brightness. Stir thoroughly to incorporate.

Taste the syrup: it should be noticeably sweet with a clear citrus/vanilla note. Adjust balance if desired — add up to 1 tablespoon more citrus for more acidity, or up to 1/4 cup more sugar dissolved in a little hot water if you want it sweeter (re-cool before serving).

Strain the syrup through a fine mesh sieve into a clean container to remove solids (zest, ginger fiber, seeds). Refrigerate the syrup until chilled, at least 30 minutes. The syrup can be stored in the refrigerator for up to 2 weeks.

Carbonation and serving

Mix the soft drink

Fill four serving glasses with ice (about 1/4 cup ice per glass if using total 1 cup). Pour 1/4 cup (60 ml) of the flavored syrup into each glass. Top each glass with 1 cup chilled club soda (4 cups total divided among glasses). Stir gently once to combine, taking care not to flatten the soda.

Garnish each glass with a slice of citrus and an optional mint sprig. Serve immediately while fizzy.

Serving note: The syrup-to-soda ratio is about 1:4 (syrup 1 part to soda 4 parts). Adjust to taste: use less syrup for a lighter drink or more for a sweeter, more intense soft drink.