I have been to Hawaii about 9 or 10 times, and I agree: Oahu is probably my least favorite of the four major islands.
There are a couple of truly great things there:
** Hanauma Bay snorkeling and some SCUBA, but it can get to be a bit crowded
** Pearl Harbor if you are a WWII history buff, and yes you will be stunned at the Memorial
** Same for hiking inside Diamond Head, up to and into old WWII embattlements, actually look across and down on Honolulu
** Bishop Museum, great history of Polynesian culture and how they explored the South Pacific using only the stars as navigation guide
If you head to Maui, I would skip the Road to Hana, mostly because it is about a 3 hour drive each way. And there really is not that much to do once you are in Hana.
** Instead make a very early morning (2 or 3 am) drive up to the top of Haleakala, and watch the night sky light and it's stars you have never seen it before (due to zero light pollution and the altitude), and then watch the sun rise over the two mountains (Mauna Loa and Mauna Kea) on the Big Island of Hawaii to the East. Bring your camera. Wear really warm clothes and bring a blanket (to wrap around you), because it is really cold up there. Eventually as the sunlight appears, you will also see the crater in front of you, bigger than the island of Manhattan. A near spiritual experience. You have never seen the sun rise before until you have witnessed it from the top of Haleakala. They have had bike rides down the mountain, but that has been on-again and off-again (dress in layers if you do that).
** Not be missed is Momma's Fish House on the Northshore. Reservations are strongly suggested. This was Frank Sinatra's favorite place to eat...in the world.
** Some great snorkeling on far South Maui, "Magic Lagoon" (park near telephone pole #18, then hike across the lava for about a mile) and "the Aquarium".
** There are some great luaus with dancing displays from 5 ethnic Polynesian cultures on Maui
IMHO, Best books for reference are "Maui Revealed", "Hawaii the Big Island Revealed" and "The Ultimate Kauai Guidebook".
There is a day-trip boat over to the Island of Lanai from Maui. The new owner of the island has just closed most of the hotel rooms.
You will not see any whales, too late in the season. Try again in November.
If you head to Kauai, there are a lot of things to see and do.
** If you are relatively fit and the adventurer type, there is open sea Kayaking around the Na Pali Coast, about 17 to 18 hours of adventure on the sea. The guide will take you into caves and by some ancient areas that used to be home to thousands of native Hawaiians. #2 as best outdoor adventure by USA Today. You will get wet. And tired, very tired.
** If you want a more sedate version, you can take a boat around from the Southwest end of the island and cruise by Na Pali coast. DO NOT VISIT KAUAI AND MISS NA PALI COAST.
** There are some incredible sight-seeing helicopter rides from near the same Southwest corner of the island. Great views of landscapes, waterfalls, canyons, etc. You will dream (pleasant thoughts) about your flight the rest of your life. Yes, it is expensive. One of the few things in Hawaii I have really splurged on, and never looked back. Dress warm, wear a securely tied-on hat and take the (faster and smaller) helicopter without doors for best photos. Very windy and cool temperature inside, but worth it.
** Kauai is the only island you can take a kayak on a river
** North shore is not to be missed, some great beaches and snorkeling. Lots of hidden beaches, if you are willing to do a bit of hiking.
** Hanelei Bay
** Bird sanctuary and Lighthouse at Kileuea Point, also up the road is a great spot to watch the sun set
** Waimea Canyon (as long as the weather is clear). You can view island of Ni'ihau (Forbidden Island) off in the West.
** Lots of water falls through-out the island
** Since Kauai is the Garden Island, try some Gardens: Allerton, National Tropical Botanical Garden, Limhuli Gardens, wear mosquito spray
** Monk Seals and their pups on the beaches (do not approach or disturb)
If you head to Big Island, there is too much to do in one week.
** Volcano Park, with live Volcanos, best time for viewing active volcanoes is before dawn or after dusk. Beware of the VOG.
** Visit Observatories @ top of Mauna Kea, also buses have telescopes that look deep into the universe after they take you down to 9,000 above sea level. Buses will provide you with really warm top coats, hoods and gloves. Wear your own boots. It snows up there (a lot), lots of people come to snow ski after fresh snowfall. IIRC, there are 12 or 13 observatories up there, and each one cost over a Billion Dollars...
** Captain Cook monument, and yes you can kayak across the bay to get up close to the monument. There is a maker where Captain Cook met his fate, and very good snorkeling there. Another way to the monument is down and back up a fairly steep road (I have not tried that route).
** LOT of interesting beaches, some black sand, some green sand... some hard to get to, but worth it. Yes, you will see and swim with sea turtles here. Do not try going to several of these beaches unless you have a jeep or 4WD, because you will get stuck.
** City of Refuge. Tons of authentic (preserved) Hawaiian heritage is here.
** My favorite beaches: Green Sand Beach, Makalewena, and a few dozen more...
** Chance to go out and deep-sea fishing from back of boat on Kona coast. If you catch something, the boat keeps it, but you will have photos of a surreal 600 lb. Tuna or some other fish (and bragging rights).
** Visit Coffee and/or Macadamia Nut plantation
** Great Garden there also, near Hilo
Either Kauai or Big Island are my favorite Islands.
Except for Oahu, I typically rent a Jeep (zipper top), just take everything with you. A Subaru Outback or similar vehicle is just as serviceable.
Oh, and I typically use VRBO.com to get deals on Rooms, cottages, apts. or condos.
The first time I went to Hawaii was the only time I booked a traditional hotel there.
Seems like on a full charge that'd be enough for a couple of laps around the island... easily last a week I'd guess.
