Muddled Fruit or Vegetables
Muddling in some soft fruits, such as berries, before adding your other ingredients is a wonderful way to add some extra flavor to the mix. Vegetables such as cucumber, celery, or peppers can also work to switch things up. Harder vegetables and fruits are better suited for adding in as a juice. For all of these, experiment with how much of the ingredients are used per drink.
Ideas: Strawberries, blueberries, pineapple, raspberries, watermelon, cucumbers, jalapeno peppers, or any kind of citrus.
Fresh herbs can really liven up a classic G&T, adding a little more seasonality to the drink. The herbs can also draw out different flavors from the gin. Try combining some huddled herbs along with citrus, berries, or cucumbers
Ideas: mint, basil, thyme, cilantro, lemon verbena, sage, lemongrass, rosemary
Add a Liqueur
Adding a half ounce or so of your favorite fruit, floral, or herbal liqueur is a fun way to experiment to find a new and unexpected flavor combination.
Ideas: Elderflower liqueur, bittersweet liqueur, Amaro, orange, berry, apricot, peach
You likely have a jar of some kind of jelly/jam/marmalade at home, give it a try in your next G&T. You may want to shake with the measure of gin and use a fine strainer.
Ideas: strawberry, apricot, orange marmalade, raspberry, or blueberry
Spices have a lot of flavors packed in them, you'll probably want to crush or grind them up prior to adding to the drink. Like herbs, they're a great way to add some seasonality to the drink or amplify the flavors in the gin.
Ideas: cinnamon, allspice, ginger, anise, peppercorns, cardamom, nutmeg, coriander seed, or cloves
Bitters of all kinds and flavors are definitely a great way to upgrade your Gin and Tonic, with just a couple of dashes.
Ideas: orange, aromatic, grapefruit, peach, anything else you can find to try