Some details to consider is that Fiji Airways does not store any bags during your layover even if 'checked through' to your final destination. Our bags were tagged to Honolulu, but we still had to collect them to go through customs and immigration. There was no bag drop for the airline to keep them during our 12 hr layover. Since we had our daughter's car seat checked in, that could have worked out well if we decided to hire a car. If you are travelling with lots of luggage you will have to consider this if venturing out of the airport.
There was a baggage storage place at the airport, which charged about $15 FJD per bag if you do not want to go exploring with your checked luggage. But factor in the time of bag handling, storing, collecting and re-checking in.
Some hotels in the area offer free airport pickup or shuttle service for a fee. We got collected for free from one of the 'backpacker hotels' located on the other side of the airport. So we just decided to take all our bags with us to save on the baggage storage fees.
Do NOT change money at the airport as there are only two places and they both offered horrible exchange rates (about 12% less than interbank rate)! The first "airport branch" is associated with Westpac bank and gives a horrible exchange rate and also charges a $10 FJD service fee. This is just after leaving customs, but before entering the arrivals hall. The second looked like an independent money changer just when entering the arrivals hall on the left. They offered western union services and had almost the same horrible exchange rate, but no service fee. I would definitely recommend using one of the ATM machines to withdraw money in the arrivals hall instead. Even after any ATM fees, I would say you are much better off than exchanging cash... even for a small amount to explore for a day.
We booked a basic room, spent a good part of the day at the hotel's beach on hammocks, swimming in the ocean and hotel pool. Then had a nice nap and massage before our long flight to Hawaii. The taxi used their meter (always request this) so the fare was less than $10 FJD back to the airport and we even shared the taxi with another traveler with similar flight times.
The hotel, food and taxi back to the airport actually cost us less than the rental car booking! (The massages cost us $50 FJD each per hour, so that bumped us over, but we had a nice relaxing, stress-free day at the beach.)
Some people take tours right from the airport. There were plenty of taxi drivers and people hanging around the airport, so that shouldn't be too difficult to sort out without pre-booking.
Others might decide to take taxis to the town-center of Nadi or the marina. Then take point-to-point taxis as needed, which would still be less than a rental car unless going far outside of Nadi.
There are a few rental car places at the airport and I would recommend booking in advance if you are thinking about this option. This could be more cost effective than tours if there is at least 2 travelling together. Just remember they drive on the left in Fiji and consider how tired you might be after a long flight.
Or you could book into a modestly priced to expensive accomodation for the day to relax and use their facilities. There is a hotel across the street from the airport that caters to long layover passengers. Our beach accomodation was still less and we got free airport transfer included. The Hyatt costs around $100-150 USD per night, is located around the marina area and reportedly has nicer beaches than nearer to the airport. I read there is black sand beach near the airport, but would say its just darker sand, not black. I'm sure there are better beaches in Fiji further away, but this option suited us fine.
So you have a few options to consider for a long layover in Fiji!
Enjoy your travels!