Bioluminescent Bay, Vieques
Concierge.com's insider take:
When the sun goes down—and before the moon comes up—head to Mosquito Bay, one of the world's brightest bioluminescent bays. When disturbed, billions of dinoflagellates (tiny, harmless organisms, more than 700,000 to the gallon) make the water sparkle. Dive in: You'll see your body outlined with a blue-green glow. Numerous island operators offer bio-tours. Island Adventures is one of the best, as its electrically powered double-pontoon boats don't pollute the water (787-741-0720).