North of Waimea on Kauai, the Haele ele Ridge Road is a 10 mile out and back route open to mountain biking. This red dirt road through the Puu Ka Pele Forest Reserve can be challenging since the elevation ranges from 3,150 to 1,300 feet, and it can be quite slippery when wet. There are several side trails and roads worth exploring.
To reach the trailhead from Waimea, head north on SR550 (Waimea Canyon Drive or aka Kokee Road) for 12 miles. At milepost 12, turn left on Haeleele Ridge Road sign. Head west on winding dirt road for 1.5 miles past the Lua Reservoir. Park near intersection with Contour Road.
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, Haeleele Ridge Road Trail is the 12th most popular mountain bike trail of all 22 mountain biking in Hawaii.