For the tequila-soaked pineapple: Put the pineapple in a large resealable container and pour the tequila over it. Refrigerate for at least 8 hours.
For the cocktail: Put ice in a rocks glass and add 2 ounces of the tequila from the tequila-soaked pineapple. Top with the sparkling water and a splash of grapefruit juice. In a separate glass, coarsely muddle a piece of the pineapple, then add to the rocks glass. Garnish with a slice of grapefruit, blood orange or pineapple if desired.