In a small saucepan, bring the water and sugar to a boil. Let boil for 30 seconds, remove from heat, and let cool completely. Once cooled, add to a blender or food processor with the cantaloupe cubes/balls. Blend until completely smooth.
To make the drink, add ice to a cocktail shaker along with a fresh mint leaf. Add the simple syrup and gin, and shake for 30 seconds. Fill glass with ice, a few melon balls and cucumber slices, and then pour in the drink. Top with seltzer, stir to combine, and garnish with mint, and extra cantaloupe/cucumber slices!