In a small pot, combine water and sugar over medium heat. Once it begins to boil, reduce heat to low and cook for 10 minutes.
Add cantaloupe, mint, and cucumber and stir. Let them steep in the syrup until cool.
Pour mixture over a fine-mesh sieve, over a cup or bowl. Use a spoon to press against cantaloupe, cucumber and mint against the side, to get the flavor out. Discard anything that doesn’t go through the sieve.
Add ice to a cocktail shaker. Add about ⅓ cup simple syrup and vodka and shake.
Strain over an ice filled glass. Top with seltzer.
Garnish with cantaloupe balls, cucumber slices and a mint leaf.