A friend asked for suggestions for creating a cocktail garden, so I recommended The Drunken Botanist by Amy Stewart. Of course, all kinds of herbs, fruits, vegetables, and edible flowers have been used in cocktail-making and many can be grown in DC.
Spearmint is so popular for cocktails that recent cultivars have been given names such as 'Mojito' and 'Julep.' I love using purple basils such as 'Red Rubin' and 'Amethyst' to make pink-tinged basil lemonade. I find pineapple sage to be particularly tasty in sangria. If you homebrew beer, you can grow your own hops.
Favorite garden berries such as strawberries, blackberries, raspberries, and blueberries are wonderful for making infused spirits, cordials, and syrups. Blackcurrants, bush cherries, prickly pear cactus, and Russian pomegranates (make your own grenadine!) also do well in smaller gardens. If you have a fence or trellis, grow hardy kiwi, hardy passionfruit, grapes, and melons. While citrus won't survive DC winters, Meyer lemon and calamondin orange do well indoors in a big sunny window.
Grow garden veggies like tomatoes and chile peppers to make your own Bloody Mary mix. And pickles make great bar snacks and garnishes, so don't forget the cucumbers, okra, and green beans.
Lastly, edible flowers make beautiful garnishes too. Violas are perfect for sweet drinks while cucumber-flavored borage compliments crisp refreshing cocktails. Try freezing edible flowers and berries inside of ice cubes.