Trying to drive
First off, most of the Caribbean drives on the left side of the road. Which will, at best, have you turning on your windshield wipers every time you try and use the turn signal. At worst, it'll have you in a head-on collision as you turn into an exit. Even if you do master left-side driving, you still won't master how to drive on the islands.
Many islands have no stoplights, which means you'll need to learn to navigate roundabouts that flow clockwise. You'll also find that traffic laws in the Caribbean are mere suggestions, and things like yields and stopping at stop signs are a coin flip. On the mountainous islands, you'll find yourself traversing steep hills on single-lane roads that can best be described as "semi-paved," and when you encounter a car coming down a hairpin turn as you go up, a rush of panic will set in. It's a great way to bring a ton of anxiety into what's supposed to be a relaxing vacation, so do yourself a favor and hire a driver when you need to go places. And in case you were wondering, no, there is no Uber in most of the Caribbean.
Not carrying cash
For whatever reason, Americans presume carrying cash is obsolete, as if every business in the world is equipped with high-speed internet and access to PayPal. Not on the islands, homie. Yes, your bigger resorts, hotels, and restaurants all take cards. But when you want to stop into that funky roadside bar for a Painkiller, or eat some conch a local just reeled out of the ocean, those guys don't exactly have Squares on their iPads.
What's more, not every country in the Caribbean is equipped to take US dollars. So always check ahead on local currency, and grab a couple hundred dollars' worth before you arrive. Many of the islands run on tips, too, and if you're not topping off your bartenders, servers, guides, and porters, that Caribbean level of service will deteriorate accordingly.