An absolutely gorgeous part of the world. Va' vau that is not Tongatapu.
The water clarity is amazing. I was there in 1979 so don't have up to date info on facilities/shopping etc.
The caves along the cliffs are not to be missed, especially Mariner's Cave.
If you are offered the chance to attend (at a price of course) an Oomu (Tongan version of a Hangi) at Api's Beach, take it.
Lobsters, fish, suckling pig and octopus baked inside a paw paw.



Also their jungle juice made from crushed water melon, coconut cream and orange juice. Add various liquors which you will not be able to taste until you get to the bottom of the bucket.
Re fishing, diving and snorkelling, get local advice, obey their rules and DO NOT touch any black coral or you will be in serious trouble. They are quite environmentally conscious.
Take along a few reels of fishing line. Back in 1979 it was a very handy trading item. Hooks, trace, swivels and sinkers as well. Hand reels and lines are all you need.
Island art might be a good thing to trade for but no doubt Aus Customs and Quarantine will require fumigation.
Cook named them The Friendly Isles and the people are very friendly but beware of a drunk Tongan. They need a licence to drink alcohol.
Attending a Kava Ceremony can be a highlight. It numbs you lips and gives you a laid back feeling. Always be respectful of the people and their customs.
Enjoy the trip. It will no doubt become a lifetime memory.