I have been traveling to Aruba for years, and have never exchanged any money. Even if you need the cash, the ATM's all prompt you whether you want your money in US or florins. I have never encountered anywhere where they didn't accept our money.
Even at the grocery stores, they ring it up in florins, then hit a key and the total comes up in dollars. The "rule" of thumb there is, if you see something in florins, divide in half and add 10%. That will give you a "close" estimate of what it is going to cost you, and they will gladly take your money. I don't know anyone that exchanges their money.
I have not stayed at LaCabana, but I know people who have and they all say the same thing...DON'T buy their meal plan...their restaurants on site on not very good, and Aruba has some excellent restaurants that you wouldn't want to miss out on trying.