• Passport - everyone needs this so this is a no-brainer.
• Comfortable Shoes - if you want to explore the interiors of island paradise (definitely required for waterfalling), comfortable walking shoes are a must, but hiking boots are probably better. If you know you'll be crossing streams or walking in water and don't want to ruin your shoes or hiking boots, water shoes with grip (e.g. river shoes, keens, tabis, etc.) are a good solution.
• Hat - the sun is intense in the tropics so don't take for granted the dangers of UV radiation. A hat will at least keep your scalp from getting severely burned (especially since the South Pacific Islands aren't all that far from the ozone hole in Antarctica). If you're wearing a broad-rimmed hiking hat, it could also help protect your neck, ears, and face.
• Sunscreen - again, given the sun's harmful UV rays, it's a good idea to protect other exposed parts of your skin from sunburn.
• Sunglasses - prevents cataracts or other harmful effects of prolonged exposure to the sun's ultraviolet rays on your eyes
• Long Sleeved Clothing - even though the tropics tend to be hot and humid, long sleeves might be a better alternative to wearing chemicals such as DEET to ward of mosquitoes.
• Rain Coat - this will keep you and your equipment dry for the most part (with a rain poncho covering the pack in addition to the body). Since rains easily come and go in the tropics, this will at least keep you sane under wet conditions (not to mention limit your exposure to hypothermia nor ruin your equipment).

Some other things worth bringing to enhance your experience include...

• Hiking Sticks/Trekking Poles - this is useful for longer and more involved tramping excursions, they provide you at least 1 or 2 extra "legs" to maintain your balance on stream crossings or alleviate shock to your knees if you're carrying a pack. They especially come in handy if the footing is slippery (not all that uncommon in tropical environments).
• Lots of Memory or Film or Portable Hard Drive - the first and third items are for digital photographers. In any case, you'll be taking heaps of photos and you'll want to make sure you can bring all your photos home

Tahiti (French Polynesia): Even though Tahiti is quite a safe and friendly country, don't be naive about crime (mostly of the petty theft variety). It does occur so do not draw unnecessary attention to how much money you have. A money pouch is good for this purpose as are TSA locks if you have to leave stuff in your accommodation while out during the day.

Fiji: There's a greater disparity of rich and poor in the Fijian Islands than French Polynesia. There has also been a history of recurring military coups (including one in late 2006), which tends to destabilize the economy (much to the chagrin of business people). Be aware of muggings or thefts, which has been known to occur in Suva and to a lesser degree other parts of Viti Levu or other larger islands like Taveuni.

Finally for all South Pacific nations, nature is inherently dangerous and unforgiving, and since waterfalling involves participating in the outdoors, you need to be cognizant of these dangers. Below is a list of things you need to be aware of to stay safe in the outdoors...

• Getting lost - always stay on the tracks and heed the signs. While most hikes are well-established, there are more primitive ones involving stream crossings or faint trails. Your best defense against this is to come prepared with rest, plenty of water, plenty of time, and a level head. But if you do find yourself lost, stop and try to backtrack to where you know you've been instead of pushing forward.
• Hear Exhaustion - given the hot and humid climate, this is a quiet killer. If you overexert yourself and your body overheats, you could suffer from heat stroke and die. To defend against this, drink plenty of water, try to avoid strenuous activity if the heat is too extreme, and prepare yourself with knowledge and time prior to your excursion.
• Hypothermia - even though it's hot and humid in the tropics, this danger sneaks up on you. Always pay attention to the weather reports before undertaking any hike that exposes you to the elements (especially in high elevations or involving streams). Bring a rain coat to avoid getting wet and losing heat that way.
• Streams and Rivers - while many popular walks have bridges, other more difficult walks involve crossing unbridged streams and rivers. Use your hiking stick to gauge depth and do not cross if it has been flash flooded or you can't see the bottom. Generally waters thigh-deep or higher are very dangerous for crossing.

In Fiji, the currency is the Fijian Dollar (FJD). When we were there in 2005, the exchanege rate was around 1.66 FJD per US Dollar. Again with the sinking dollar, today, the exchange rate is round 1.55 FJD per US Dollar. While many places will accept Visa and MasterCard, there are many others accepting only cash.

These types of destinations are really meant to be relaxing vacation destinations. And if all you're doing is a few tours here and there while spending the remainder of your time eating out and chilling out, then a weeklong trip is plenty of time. Most people spend about a week or a little longer. Of course, if you want to add a bit of adventure (like waterfalling excursions), then I'd say a week and a half is optimal though this can easily be extended for more island hopping.