North of Waimea on Kauai, the Kauhao Ridge Road is a 8 mile out and back route open to mountain biking. This red dirt road can be challenging since the elevation ranges from 3,500 to 1,800 feet, and it can be quite slippery when wet. While steep, it is actually one of the more moderate ridge roads in the area.
To reach the trailhead from Waimea, head north on SR550 (Waimea Canyon Drive or aka Kokee Road) for 13 miles. Just past milepost 13, turn left on dirt road for 0.3 miles. Just past the Methodist Camp, turn right on dirt road. Continue for 0.35 miles and stay to the right where the Contour Road intersects with the Kauhao Ridge Road. Start riding on Kauhau Ridge Road from here.
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, Kauhao Ridge Road is the 14th most popular mountain bike trail of all 22 mountain biking rides in Hawaii.