I tend to forget that we don't need so much stuff...I forget that the "what if we want to do this?" hasn't happened in 25 years of travel to Negril... Money, passports, and boarding passes are really all you need. That said, I'm an organized packer (perhaps a bit OCD passed on through genetic code) and have a fairly detailed checklist that helps.
We have found that we do like to have small air mats/floats and those are easier to take with than to get in Negril. And in Chicagoland, it's easier to find one in the summer months and have on hand for packing in the winter.
I also like MaryAnn's reminder of a pen... I usually have 3 or 4 (the OCD flairs again and what if one suddently dries up?). Someone on the plane always needs to borrow one.
I will also add to the list -- keys or some way to get back in to your house when you return... Yes, we've had first-hand experience with this one...it's on the list now. Happy packing!