About an hour or two west of Sanya city is Jianfeng mountain, a national park reserve of beautiful rain forest, clean air, and amazing wildlife. It's around a two-hour hike up to the peak (or you can try to catch a motorbike or car); once there, you can see all along the coast and all of the city.
For some reason, this is considered a secret destination in Sanya. You'll have to hire a private car or take a guide to get out here. It's well worth making the effort—you don't get the overwhelming crowds that you sometimes encounter elsewhere.