Southwest of Hilo in the Hawaii Volcanoes National Park, the Kilauea Crater Rim Trail is a great 3.6 mile mountain bike ride that combines road with rim-side trail.
To reach the mountain biking trailhead from Hilo, head south on SR11. Near milepost 29, keep an eye out for the entrance to the National Park on the left or south side of the road. After entering the park, stay to the right towards the Visitor Center and park there.
Starting riding from the Visitor Center on Crater Rim Drive heading south towards the Steam Vents. Continue on the narrow paved path on the right of the road until the military camp. Cross the road and continue on the gravel trail.
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.