Add the cubed watermelon, lime juice, maple syrup, and ¼ cup mint leaves to a high-speed blender or food processor. Blend until smooth.

Pour the mixture through a fine-mesh sieve into a large bowl or pitcher, pressing gently to extract the juice. Discard the pulp.

In a small dish, mix Tajín with the lime zest. Rub a lime wedge around the rim of each glass, then dip the rims into the Tajín-lime mixture.

Fill each glass with ice. Pour in the strained watermelon-mint juice, then top with an equal amount of club soda (1:1 ratio). Stir gently to combine.

Garnish each glass with fresh mint and, if desired, a thin slice of jalapeño for a hint of heat. Serve immediately.