Been to just about all the places referred to here. Here is my summary.
Kauai - Tunnels and Ke'e on the north side are great fun, but not as good as Maui. Also, given that they are on the nrth side, might not be ggod choices.
Oahu - See other posts.
Hawaii (Big Island) - There are quite a few good spots between The Place of Refuge and up the Kohala coast.
Maui - La Parouse is great early. The Aquarium is cool, but I went when you could hike through the Lava field (an experience itself). There is another place in the same area called "The Fish Bowl" that is very much like The Aquarium. The snorkeling is also very good right off the Grand Wailea/Four Seasons beach on the left side especially. I saw eels, turtles, octopi and three Manta Rays the last time. I was snorkelling for a while, came back in and then saw the shadows swim by. I went out again later, thinking they were gone, but they ended up swimming about six feet below me. My wife said she saw them approaching me and was freaking out. That was extremely cool. Molokini is a must do at least once. Maui Dive Shop has an excursion that goes out of Kihei Boat Harbor for both Divering and Snorkelling. The other outfiut out of Maaleaa (sp?) harbor that I like is the Wailea Kai. Honolua bay is also a must see if the weather has been good. It is north of Lahaina, so it takes a good 45 minutes to get to from Kihei. I prefer staying on the south side (Kihei/Wailea) as it is a bit quieter and not as touristy.
If you really want to do some homework, there is a series of books that are excellent. They are blue in color with a picture of the island they are about on the cover. Go to
www.wizardpub.com to see them, but they are available in most bookstores. They are very good books. They are also extremely honest in their descriptions. They also have pretty good snorkel maps. The area referred to above as "Five Caves" was also called "Five Graves" and that shark is well known. If you take the Road to Hana, have the book in hand as it will tell you about some pretty off the beaten path neat things. We went to see one waterfall and were the only people there until we were hiking out and a couple came in holding thier copy of the same book, got a good laugh out of that. Also, if you are up on Haleakala and looking for a good hike, park in the parking lot at the 8000' elevation and there is a trail that looks like it doesn't go anywhere. Follow it for about 15-20 minutes and it will open up on the side of one of the fingers that projects into the crater. This trail does dozens of switchbacks until finally reaching the crater floor.
ANyway, have fun. I'm jealous.
...if you can get there 
