Kulani Forest, Friday ride rain or shine every Friday, Chris at Hilo Bike Hub is the main culprit, generally don't ride MC's in there out of respect to the MTB crew that maintains the trails. The OHV park is right up the road and we're opening up some routes up to it about 7-8 miles up the road.
Hawaii Volcano Nat'l Park, the mellower trails are now open to MTBs absolutely no MCs, in the old days we'd do Ninja MTB Runs through the Park Hiking trails, wife works there now so no civil disobedience for me...
Mana Road, basically a firebreak type road that goes along the eastern side of Mauna Kea, OK for motorcycles, can go WFO!!!!
Beach Road is pretty good and leads to other good opportunities for MCs, mellow for MTBs.
Old Puna Road if you can find it is brutal after the first mile and a half, I've never done it but heard stories of guys getting helicoptered out after spending the night... Check It Here
Mud Lane just east of Waimea is nice single track for MTBs and MCs.
I'll think of some more and let you know.