Cocktail onions and olives keep forever in the fridge. Even if you don’t use them often, keep some around. Cherries are also a must, but find some good quality ones that you like. There are enough good cherries on the market that there is no need to have any business with the neon red ones anymore. It’s even easy to make your own.
So many drinks benefit from a piece of lemon rind that it’s worth keeping a lemon on hand at all times. Oranges and limes are nice to keep on hand too, but not as essential.
One item you might not think to buy is Tomolives. These are little, pickled green tomatoes the size of an olive. They’re great in a Martini, or even just as a bar snack.
Lastly, grow some mint in a pot. Don’t grow it in the ground because it will spread like a weed. But it’s easy to grow and great to have around for the bar.
Bitters are a key ingredient to keep on hand for making cocktails. Any given cocktail with bitters will likely call for only a couple drops. This might seem like too little to make a difference, but you’d be mistaken. A couple drops of bitters can totally transform the way a cocktail tastes and the way the different flavors in that cocktail play together. Bitters won’t actually make your cocktail bitter (unless you add a ton), but rather it adds a background complexity and tends to marry different elements, like sweet and sour. If you aren’t very familiar with bitters, make a simple cocktail (perhaps a whiskey sour) and divide it into two glasses. Add a dash of bitters to the top of just one glass and taste the difference.
We are lucky to live in an era where there are countless bitters on the market by manufacturers big and small. I highly recommend you explore the possibilities. However, if you want to take the guesswork out of it, it’s simple. Buy a bottle of Angostura and a bottle of Peychaud’s.