Puerto Rico...
1) You can visit the rain forest (El Yunque is how I think it's spelled). They have a great waterfall that you can swim under that's on a nice hike after a beautiful drive.
2) Culebra. It's one of the spanish virgin island and is supposed to be beautiful. The water and beaches should be just what you are looking for.
3) Vieques. It's another spanish virgin island (one I've been to twice) and has beaches that aren't crowded at all. Furthermore, they have the biobay that you can kayak in at night and the water glows when it is disturbed. Very cool. So cool in fact that I took my wife there to show her it even though I'd been before.
http://www.viequestravelguide.com/mo...scent-bay.html
4) It's still in the US, so you don't need a passport and it's generally cheaper.