Ditto recommendations on snorkeling tunnels. BUuuuuut, my brother just got back from his honeymoon on Sunday, and the swells are already starting to roll in for the fall/winter season. It's not the end of the world, but something to be mindfull of. If the trades are blowing on the days you want to go out, the West Side of the island may be a better bet.
Definitely do the Na Pali Coastline tour via catamaran or zodiac. The zodiac tour can get a little agro, (especially if the swells are up) but they're allowed in closer to the shoreline than the larger boats, and as such you'll get to see some caves and the like. The beach they drop you at for snorkeling and lunch has a decent reef formed from an undersea freshwater spring that creates a nice path through the reef. This allows you to see so dropoff without being on the perimiter of the reef. We did this trip 2 years ago and wish we could go back just about every day. My brother and wife got extremely lucky and actually inadvertantly got up close and personal with a monk seal. They were snorkeling in the reef on the Na Pali coast and his wife got a driveby from the monk seal. She grabed for the camera, but all she got was a shot of his head to the side of the frame. Since you aren't suposed to be within a 100 feet (if I remember right) of them, they didn't pursue for more pics. Anyhow, the Na Pali tour usually gets you some up close time with dolphins and sea turtles as well.
There is also good snorkeling off of State Park Beach north of Port Allen, but you have to drive out a dirt road to get there. If the swells are up though, once again things change.
As for the meth issue raised earlier, it's a real concern but just take typical travel precautions (store your valuables in a safe in the room, or with the concierge) keep your stuff locked up, and don't show off anything fancy that might lure trouble your way.
Have fun on your trip, and if you end up buying one of the reusable underwater cameras from Wal Mart, the ABC store or SeaSport Divers in Poipu, ditch the film that comes with the camera as you don't know how long it's been in there. Also make sure you get any film you take developed before you leave. Supposedly the airport security stuff shouldn't hurt anything slower than 1000, but I would hate to risk my photos to chance.
Chris