Tongatapu is the main island of Tonga in the South Pacific. There are some pretty beaches, though not as good as in Fiji. The main feature in Tonga is a rich historical background. It is believed, that the first Polynesian settlements were somewhere here. Shore and seabed: sand, rocks. 
Tongatapu island hosts the most of hotels in Tonga. The best beach resorts are located on small islets just north off the main island. However, most of these islands don't have any accomodation, and are available only as a day-trip destination.
The Tonga's capital Nuku'alofa is located in the north of the island. It has the best choice of hotels in the country, and accommodation rates are very cheap by the stardards of the Oceania.