What's The One Thing Every Bar Cart Needs?

My mom called me a few days before my birthday last week. "I'm sending you a package," she said between slurps of Diet Dr Pepper. "Apologies in advance, because it's heavy as hell." The roughly 40-pound box arrived the next day. I lugged it up the stairs to my third-floor walkup, opened it up, and found a gift I never knew I needed: a bar cart. I assembled it, arranged it in my dining room, and stood back to admire the fruits of my labor. There was just one question: how do I stock this thing?

Don't get me wrong; I love a cocktail, and I can whip up something semi-decent if necessary. But I hardly ever drink liquor at home, and my whiskey palate is sorely lacking. I've never needed a bar cart, but now that I have one, I'm a little unsure where to start.

Of course, I've already consulted our comprehensive guide to building a bar cart. I already have a few essential odds and ends, including:

• A set of very nice assorted cocktail glassware that I weirdly inherited from my grandfather when I was nine
• A bar spoon, a shaker, a strainer, and a jigger, all of which came with the cart
• One (1) airplane-sized bottle of Fireball Whisky, for my enemies
• Several sealed bottles of tequila, one of which I accidentally stole from a hotel (long story)
• A decent bottle of gin (I have no idea where it came from)
• My Little Boys, which is what I call my eight plastic beverage markers shaped like Speedo-clad men

My question for you is this: if I wish to transform into a savvy home bartender, what else do I need? Actually, let's make it even more specific: what's the one thing I need to acquire immediately to salvage this bar cart? It could be a cocktail ingredient, a gadget, a framed photo of Donn Beach—whatever I need to have a proper go at this thing.