Which Islands are you visiting?
Last time we stayed a week on The Big Island and week on Ohau (where we got married). and a third week in the US doing LA/LV (hated) and Area 51 (scarily good fun)
This time we are doing The Big Island again and either Maui or Kauai - think leaning towards Maui to be honest (better weather, but less stunning than Kauai - ooh decisions)
If you go to the Big Island (we stayed in Kona) you should visit the National Volcanoes Park - that is amazing be prepared to hike tho!) drive round the Island and you will drive through so many different temperate zones that its just mind boggling. One minute your driving across larva flows, you turn a corner and you are driving past a mature rainforest, or ski on the snow capped mountains near Mauna Kea, Also, there is a hotel called the Kona Surf resort where you can go with a smoke on a nighttime and watch the Manta Rays feeding off plankton in the sea to the side of the hotel - that was awesome. We also went snorkling to Captain Cook's bay (on a boat trip) which was lovely too. Drove round to Hilo but found that quite built up and industrial - not that impressed to be honest....
On Ohau, we went to Pearl Harbour to see the museum and took a boat to the Monument in the harbour showing all the names of those who had died on the ships - that was something very moving. We then went on to Honolulu for shopping - some great stores. Also went to Diamond Head Crater, an extinct Volcano which had the most spectacular views of Waikiki Beach.
There's also whale watching, hiking, diving, helicopter rides over the active volcano flows and of course surfing lessons to be had also.
I have to say - get the Lonely Planet book on Hawaii - we found it invaluable for up to date information on hotels, travel, restaurants, stuff to do and see - well worth the money.