In a small saucepan, combine the water and sugar and heat gently, stirring just until the sugar fully dissolves; do not boil. Remove from heat, add the lime peel and jalapeño slices, and let steep for 10–15 minutes until aromatic with gentle warmth, then strain and cool completely.
Add the chopped cucumbers and ½ cup cold water to a blender and blend until completely smooth; if the mixture is too thick to pour, add water 1 tablespoon at a time — cucumber flavor must stay dominant. Strain through a fine-mesh sieve into a pitcher for a clean, watery-fresh texture.
Stir 60–80 ml of the cooled lime–jalapeño syrup into the cucumber juice, then add fresh lime juice gradually, tasting as you go; the drink should be crisp and green, not sweet and not sharply acidic.
Fill glasses with ice, pour over the cucumber agua fresca, and garnish with thin cucumber slices and optional mint leaves; serve immediately for maximum freshness.
