What to bring !
The Backpack should be right for your weight and should be waterproof.
Eventhough, in most of your accomodations they offer mosquitonet I recommend bringing your own since the nets are often damaged or they do smell horrible.
Bring comfortable shoes since you might walk a lot especially in woods and wet arreas.
You will definetly drink a lot all around the day. For your trips you need a good and unbreakable bottle.
Consider bringing triband or more mobile phone.In the backpacker accomodations on the islands you will not find any phones. Anyway, network will be hard to find on the islands.
There is nothing much to do on the islands exept from relaxing. Music can be a priceless thing.
If you bring your mp3 player, your camera or even a mobile phone you will need one of these.
To kill the time and get in contact with the Fiji babys it is helpful to bring some games.
Make sure you bring the medicine that you need to take. On the isalnds and most o the other places you will not find any pharmacys at all.
Beforehand, you should bring enough money. There might be the chance to get money only in Nadi or Suva.
Bring enough swimsuits. You will wear them most of the time.
You will need something to cover your head since the sun can be pretty strong.
Bring enough to be ready for a whole day of sun.
You can rent this on almost every island and place but having your own makes it a lot easier since thex can be very disgusting from time to time.