Nadi - Kinda 3rd world county setting, you don't want to stay there. 20 minutes by cab from airport. 35 minutes is the Denarau area.....Big chain hotels, golf course, high end housing, harbor and where the boat/seaplanes leave for the outlying islands. Big pricey hotels:
The 5 Best Denarau Island Hotels on TripAdvisor - Prices & Reviews for the Top Rated Accommodation in Denarau Island, Fiji
Several beach resorts on the main island; some nice, some not so much. I checked out one I had almost booked. The dirt road in was so bad, I took out the transmission pan on my Chinese Chit-box rental car.
I walked thru several of these resorts; Had a good BBQ on the beach at First Landing:
http://www.tourismfiji.com/fiji-map-resorts.html
What I did and would do again: One hotel across from the airport; Raffles Gateway:
Raffles Gateway Hotel, Fiji - Nadi Fiji Islands Holiday Packages, Diving Scuba Fiji, Weddings, Tropical Sandy Beaches, South Pacific Islands Fiji
Was around $100/nite 2 yrs ago. Explore by rental care by day, take a boat ride out of Dennarau for the day. We checked out resorts we would have stayed at if we had been there more than 4 days. We had a great BBQ dinner and Native show on the beach at the "First Landing" resort.
If you want to drive all the way around the island, I'd plan 2 days. Stay on the main highway. The roads that criss-cross are 4WD with high clearance required. Bula