Since your flight will arrive in Haikou, I think you could stay 2 nights in Haikou firstly and then 2 nights in Sanya. At the end, you could take a flight back to Singapore from Sanya Airport.
As I know, the Singapore passport holders could visit Hainan without visa but they should participate in a tour group. So, if you travel individually, you have to apply for China visa in advance before coming to Hainan.

Thanks Jenni! I've bought 2 ways return air tickets from Singapore-Haikou-Singapore. My flight arriving on Sat 16:30 and depart on Wed 17:00. So, what would you suggest for my stay in Haikou and Sanya? I will get Visa from Singapore before I go Haikou.

Well, it doesn't matter. On the last day, you could hire a cab driving you from Sanya to Haikou Airport directly. The fare is about CNY270~300. The journey is about 3~4 hours, so you could leave from Sanya around 9am.

Can anyone comment how is the Sheraton Haikou? Is the location close in city centre and easy access?

It is not in the city center, but not very far, in Nanhai Dadao. It is within 20 minutes's drive to the city center.