Been to all the islands. Last trip we did my fav- Kauai- and the Big Island- which used to be my least fav and moved up to my second ahead of Maui. We took our son.
Volcanoes National Park is an absolute must, as is a trip up Mauna Kea- the most incredible experience ever. We did a sunset/stargazing tour with a group- expensive- but worth it.
http://maunakea.com/
Also- beautiful beach there is Hapuna Beach often called the best beach in the USA.
Kauai is just so great and laid back- the snorkeling. Loved salt pond and Lygate State Park beach and Kea beach and the Napali Coast (took a boat tour). We found Lumaha'i Beach with views of Bali Hai. (dangerous to swim in, though).(location of the scene from the old movie South Pacific- song-"Gonna Wash That Man Right Out of My Hair"..and song "Bali Hai")
Then there is the Fern Grotto and Waimea Canyon.
Maui is also beautiful- you have the National Park there as well.
Oahu- the second time there we just did Pearl Harbor again and a short excursion into Honolulu on a flight layover on the way home from our stay at the two other islands.
I recommend doing at least 2 islands and staying 2 weeks. Our last trip was 21 days. We exchanged and actually tacked on a rental in between to make it work.
Put in a request asap- when I did it I think it was about 2 years in advance. This was before the creation of TPU's. We used our fixed RED week and our White summer week with a few days rental in between at the resort we were staying in in Kauai to make it work until our check in on the Big Island.
On the Big Island we stayed in the desert area- I didn't care for the Kona area.Wanted to be near Hapuna Beach. In Kauai we stayed in Lihue- I like the dry side as opposed to the Princeville side which can get more rain.
You need to rent a car. Kauai is small and easy to drive one end to the other- but a lot of traffic as it is only one main road. The Big Island is- well- BIG- and be prepared to do a lot of long driving
First time we went in October and second time we went in July. Both times had beautiful weather. Keep in mind in Hawaii it gets dark by 5pm- so it is not like summer is here where it is daylight until 9pm.