Kona
My review will primarily discuss the manta ray night dive in Keauhou Bay, close to the Sheraton Kona Resort & Spa. This is a very unique, once in a lifetime time of dive that I highly recommend. We got on the boat prior to sundown and headed out to a secluded bay where the manta rays tend to congregate. When the sun sets, the boat lays out floats on the surface of the water with large downlights to attract the plankton, which in turn, attract the manta rays. Our boat had a combination of snorkelers and divers. The water is shallow (7m) enough that snorkelers can enjoy the manta show, albeit from the surface of the water. We had 2 mantas join us for the duration of the 65 minute dive, doing acrobatics right before our eyes. I was using a 12-24mm lens on a cropped sensor DSLR, which was barely wide enough. If I have the opportunity to do this dive again in the future, I will most likely use the Tokina 10-17mm fisheye + 1.4x teleconverter.
I understand that manta sightings at Keauhou Bay are frequent but not guaranteed. We saw 2 mantas during our dive; on a good day, many more show up. If you have the bad luck of not seeing any mantas, I believe that most operators will give you a free pass to come back and try again another night. Bearing this in mind, it is advisable not to do the manta night dive on your last night in Kona, just in case the mantas don't show up.
Kona is fantastic for topside activities. The island's landscape is amazingly beautiful, with a combination of volcanic rocks and lush greenery (I believe Jurassic Park was filmed in Kona). Various outdoor activities such as watersports, hiking, dolphin tours, etc. are easily available. And the laidback attitude prevailing in Kona is wonderful and great for the soul.