Cucumber and grapefruit juice combine for ultimate refreshment in this happy hour cocktail. Jalapeño adds just a hint of heat.
Rub the rim of a highball glass with 1 slice cucumber; dip rim in salt. Fill glass halfway with ice and add thin cucumber slices.
In a cocktail shaker, muddle the remaining 3 thick slices cucumber and the jalapeño slice in tequila. Add grapefruit juice and agave nectar; add additional ice. Cover; shake until very cold. Strain into prepared glass and top with club soda. Stir gently; garnish with additional thin jalapeño slices if desired.