Apart from a few historical sites on Tongatapu most things to do in Tonga reflect its island nature. Diving, snorkelling, fishing, boat trips, kayaking and kite surfing are all possible. There are some lovely beaches if you just want to laze around. Tonga has some good restaurants and this is the place to come if you like lobster.
Take time to learn a little about Tonga's fairly feudal culture and its many traditions. Go to church. Even if you are not religious the singing can be very moving. Watch tapa cloth being made from mulberry bark and try a drink of kava, the traditional drink, which is a mild narcotic.
If you don't work you don't eat. Tongans don't want to hear that it's hard on the coral beaches lined with palm trees and emerald lagoons. There are many opportunities for skilled trades from the streets to the shops, in the schools to the churches and yes from the markets to the office. This is a hot spot for skilled navigators spanning throughout 169 villages and 150 islands. Some major exports include Vanilla, handcrafts and specialty pumpkins grown for export to Japan. Other agriculture sectors include root crops like taro, tapioca, sweet potatoes, yams, coconuts, bananas, mangoes, papayas, pineapples, watermelons and even peanuts.
If you are on a visitor's visa, you cannot be involved with business or take up employment while in Tonga. You also cannot take courses from an educational institution. It is mostly illegal to try to change a visitor's visa into a visa that allows for employment, so if you intend to have a job while in Tonga, make sure you have an employment visa in advance. Apply for your visa at least one month in advance. If you are already in Tonga and would like to extend it, contact the immigration department one month in advance about the extension.
While employment visas are technically available, the immigration department will probably be reluctant to granted you one as Tonga has a high unemployment rate, and would prefer that jobs be taken up by Tongan citizens as opposed to outsiders. If you're coming to Tonga for humanitarian or volunteer work, you need an employment visa for that.