Are you serious? What I recommend is that you step away from me as rapidly as you can, before I whack you in the head with a snowball. Poor you! Going to the nasty Caribbean where its relatively dry and night temperatures are usually 75! Oh, the humanity!
I mean, obviously there wont even be a need for a sleeping bag. Really all you need to take is a cotton or silk sleep sack. Cocoons Cotton Mummy Liner ($20; cocoonusa.com) gives you just a touch of warmth and something to hang onto in the night, and it weighs next to nothing. The silk version costs $50 and is a little warmer. Honestly, it and a T-shirt ought to suffice even if a cold snap" sends temps plummeting into thegasp!mid-50s.
If you decided you could use something warmer, then consider REIs Kilo Flash +40 bag ($179; rei.com). As the name implies, its a sleeping bag rated to 40F or warmer. It weighs just over a pound and packs down to nothing. You could unzip it and use it as a comforter, too.
Now go away.