Okay so I just did this REALLY long post and lost it somewhere. So I am going to have to shorten this up a little bit and if you have any questions just let me know. I am going to let you know that my soon to be husband grew up on Oahu in Honolulu. Born and raised. Left when he was 18. His mother and her family also grew up on Oahu and his father is from Maui. Born and raised. I have been for a total of 4 weeks so these are my first hand experiences. I have not been to Kaua'i yet but i would like to do the snorkeling thing at the Na Pali coast.
On Maui,
Haleakala crater is magnificent! We went at like 3:00 and watched the sunset. Not fun coming back down in the fog bank in the dark though. The road to Hana is very long slow and tedious. We got 45 mins in and still had like 20 miles to go so we turned around and went home. ( I was getting carsick and I was driving.) Iao valley is really pretty and green. We also went snorkeling at Molokini. This was my favorite part of our Maui trip.
In Kona,
You should definitely check out the Waikoloa Hilton Dolphin Quest.
you should also check out the City of Refuge. Very interesting and LOTS of sea turtles!!! Also you should check out Pu'ukohola Heiau (hay'-ow). It is actually 3 heiaus and one of them is the Hale o Kapuni Heiau. This is a shark heiau and when I was there, there was actually a shark fin approx. 30 yards out. It was spine tingling.
In Hilo,
You should check out the Pololu Valley which is the birthplace of King Kamehameha I. Absolutely stunning! Also there is a bakery there called two ladies bakery. They make these mochi covered strawberries that are actually to die for!!! SO GOOD!!
In Honolulu,
You need to go to the Polynesian Cultural Center. Lots of fun. They have a luau and they have fire dancers and it teaches you about all the south pacific islands and people. Pearl Harbor should also be on your list of things to do. And Hanauma Bay has excellent snorkeling. Don't forget your underwater camera. It only cost like $3 or $5 a person. You might have to take the bus to get there though. We have also done the Maita'i Catamaran at sunset and it was a lot of fun. We even got to see the mysterious "green flash".
If you have any other questions, just send me a private message and I will answer everything I can.
Two websites you might want to check out are www.gohawaii.com and www.nps.gov. They should be very helpful as well.