Have a nice vacation! Kona is awesome. It feels a little more laid back and relaxing than some of the other islands.
We've really had a great relaxing time amid lots if activities. We did a night swim with manta rays, a 5 hour snorkeling tour, a luau, and a zip lining adventure through the tree tops. The beaches on the big island are primarily hard lava rock. Really not for detecting. I do have to admit for looking for gold rings among the fish during the snorkel. Can't completely take the pirate out if me.