I think 3 days are enough in Haikou. Well, here is a list of Haikou's attraction: https://www.travelchinaguide.com/attraction/hainan/haikou/ So, please consider going to Boao or Sanya~

The best hotel in Haikou should be the Sheraton Haikou Resort, haveing good view and comfortable facilities. For resonable price, how about the Haikou Hotel? It is located in the prosperous shopping area.

The various kinds of seafood could be found easily in Haikou, supplying at these reasonable booth to delicate restaurant.