There are places on Oahu for 4-wheeling. Xterrian mentioned one. "Peacock Flats" is another. Contact Jamie at:

hawaiianhardbody@hawaiianhardbody.com

or introduce yourself to the folks on the Hawaiian Trail Riders Message Board at:

http://www.hawaiianhardbody.com/

You won't be the only Xterra in the group, but be forewarned, these guys hit trails for the hard core 4-wheeler and are not for the faint of heart.

BTW, I have an XOC sticker on the back of my Solar Yellow Xterra, just below my Xterra Club Hawaii sticker..