Late reply I know, but I just saw this topic...
If you make it to the "Big Island" (the actual island Hawaii where the volcanoes are) a trip to the Volcanoes National Park is highly recommended. The Kilauea caldera is stunning. Access depends on how active the volcano has been lately

but when I was there many years ago you could walk across the whole caldera. It was a very surreal walk - like being on a different planet with steam vents and hot glassy rocks and the occasional roar of lava spewing from a distant vent. I saw a 4WD that had parked off the designated area and too close to some hot rocks had melted tyres... There are some great short walks around Kilauea including going through a lava tube.
Also on the big island I'm pretty sure there are walks around Kailua-Kona, which is a beautiful little place in its own right.
The island of Maui has the other big National Park in Hawaii, though there are definitely wilderness areas on Oahu as well.
Good luck with your trip - I have no doubt you'll love Hawaii.
Cheers,
Alliecat