For the Salt Rim: Set out two small shallow plates. Place salt in one dish and a thin layer of agave syrup (or simply syrup) in the other. Dip the edge of a highball glass in the syrup, then dip in salt.
For Each Cocktail: Fill the glass three-quarters full of ice. Add the grapefruit juice, vodka, club soda, and 1 teaspoon simple syrup.
Stir with a cocktail swizzle stick. Taste, then add a little more simple syrup if desired. Add a fresh grapefruit wedge to the rim.
Notes
Try a spicy Salty Dog by adding a pinch of cayenne pepper to the salt before dipping.