In the southern coast of Upolu, the beach at Virgin Cove and Sa'moana resorts are pure bliss. Such accommodation providers have very few guests, so you will still feel that you have the place to yourself.
Otherwise Return to Paradise beach near Savaia on the southern coast of Upolu is also nice, and there's no resort.
In the north, you can find a small secluded beach east of Vailele Bay. almost 7 years ago