Other Liquor - you can absolultey change the tequila to a different spirit if you prefer. Vodka, Gin, or Rum (light or spiced) are a great substitution.
Herbs - Muddle some herbs into the lime juice before shaking.
Citrus - Add other citrus juice to the lime juice. Play around and see if you can find a great mix of flavors.
Fruit - Muddle fresh fruit in the cocktail shaker before shaking the cocktail. Strawberries, blackberries, and even watermelon would be a tasty addition. (Watermelon and tequila are so good - Watermelon Margarita recipe).