Most people come to south Hainan for beaches and cocktails, which is fine. Actually, it's a blessing, because it means places like Diaoluo Mountain remain off the beaten track for the majority of visitors, and thus retain more of their unspoiled character.
If you enter through the yellow village at the main gate, you'll need to show your passport and pay a small fee to get in. It's not a steep hike up the mountain, but it is fairly long. At 5000 feet, there's a hotel with a restaurant, and a pretty lake. From there you can continue on a paved path, seemingly forever. There is at least one stunning waterfall—part of a river you may want to cross.
The temperatures up on the mountain drop very quickly, especially when there are sudden, and torrential, rain showers. Definitely be prepared for all weather.