For an extraordinary experience that amazes both tourists and locals, take a trip out to dive (or snorkel) at night with the manta rays near Kailua-Kona on the Big Island.
Guides take tours out from Honokohau Harbor in the evening before sunset, and most allow guests to get in the water in a couple places before the nighttime swim with the manta rays. Divers sit on the bottom (with rocks on their laps if they are too buoyant) while snorkelers float above watching the manta rays feed on plankton.
Several outfitters provide this experience, including Kona Honu Divers, Big Island Divers, and Jack's Diving Locker.
As swimming with the manta rays becomes more popular, so does the endangerment of these creatures. If you choose to take the plunge, make sure you book an outfitter that emphasizes the care of the rays and do not touch them when you are in the water. This depletes the protective mucus on their bodies and exposes them to injury.