For the watermelon ice cubes: Puree the watermelon, honey and lime juice in a blender until smooth. Pour into a square ice cube tray. Freeze until firm, 3 to 4 hours.
For the simple syrup: Combine the sugar and 3/4 cup water in a small saucepan. Bring to a simmer and shut off the heat. Stir until the sugar dissolves. Cool.
For the cocktails: Mix the vodka, lime juice, mint leaves and 2 ounces simple syrup in a pitcher filled with ice. Top with club soda. Put 3 watermelon cubes in each of 2 highball glasses and pour the cocktail over.