Budget rents Xterras in Hawaii, but it's my understanding they don't have any 4x4s. Some rental companies disable the 4x4 feature of their Jeeps to prevent tourists from wrecking the vehicles. My advise is to check with a good travel agent to find something with four wheel drive.
If you succeed in finding one, drive up to the 13,796' summit of Mauna Kea (4x4 necessary). There are some places to pull off the main road if you need even more excitement, but this is officially discouraged.
Stay off virgin lava, especially the type known in the Hawaiian language as "Aa". This lava is razor sharp and will shred your tires within just a matter of several yards. Bring a cell phone.
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I started off with nothing and still have some left.