North of Waimea in Kokee State Park on Kauai, the 4,350 acre Kokee State Park has miles of 4WD roads open to mountain biking. The state park contains the Waimea Canyon, which is known as the Grand Canyon of the Pacific. Its a beatiful red rock trek along canyon rim and through rain forest.
To reach the park from Waimea, head north on Kokee Road (SR550) for 15 miles.
Here are some notes on adventuring in Hawaii. First, unlike mainland forests, the tropical island forests can grow very dense and can mask dropoffs. Plus, the trails can be very wet and slick most of the year (December to February is the main rainy season, but expect rain all year round). Stay on the trail to avoid disorientation. Second, speaking of disorientation, be aware that most Hawaiian topographic maps use the Old Hawaiian Datum. When trying to use the latitude/longitude from those maps with a GPS unit, you will think you are 0.4 miles south/southeast of where you really are. Kind of important in a dense jungle. Third, the bacteria Leptospirosis is found in the local waters and mud. It causes flu like symptoms within 2-20 days of contact, and can be fatal if not treated with antibiotics. Fourth, when hiking about, walking sticks can be very helpful for stablity in the slick conditions, for probing mud hole depths and clearing webs of the tiny crab spider.
Overall, Waimea Canyon is the 2nd most popular mountain bike trail of all 22 mountain biking rides in Hawaii.