I haven’t ridden MTB so I can’t say for sure. I know people who do and generally there’s not as much MTB spots. You can to look at Olomana, I know there’s a trail there. Also St. Louis heights. If you’re looking for something easy, there is always Kualoa ranch which is very scenic but $$$ as it is private land and it’s a tour. Some of the staff at The Bike Shop Honolulu and Bikefactory MTB so ask them and they’ll be able to help you out.