Combine water, sugar, and fresh mint in a medium saucepan over medium-high heat. Bring mixture to a boil, stirring regularly to help the sugar dissolve, then let simmer 2 minutes.

Remove the saucepan from the stove and let cool for 30 minutes, leaving mint leaves in the pot to steep in the syrup.

Pour the cooled syrup into a lidded container or jar (you may want to use a small strainer for this), then discard the mint leaves. Syrup will keep in a covered jar in the fridge for 1-2 weeks.

Add pineapple, water, lime juice, and mint syrup to a blender. Put the lid on the blender and blend using the “Smoothie/Puree” function until the mixture is smooth. Taste, and add more lime juice or mint syrup to your liking, if needed.

Pour agua fresca into glasses filled with ice, then garnish with mint leaves. Serve immediately.