Make the cinnamon simple syrup: In a small saucepan, bring water, sugar, and the cinnamon stick to a simmer over medium heat. Stir until the sugar dissolves. Let it simmer for 5 minutes to infuse the cinnamon flavor, then remove from heat. Cool completely before using.
Prepare the glass: Rim the edge of your glass with lime juice, then dip it into salt or cinnamon-sugar for a sweet and salty contrast.
Mix the cocktail: In a shaker, combine the tequila, fresh grapefruit juice, lime juice, and cinnamon simple syrup. Add ice and shake well for about 10-15 seconds.
Serve: Strain the mixture into your prepared glass filled with ice.
Top it off: Add a splash of club soda for some fizz, and stir gently.
Garnish: Add a wedge of grapefruit and a cinnamon stick to give the drink a little extra flair.