To get from the Nadi airport area to Nadi town is easy. Many buses local run, it isn't far.
Not far north of the airport (opposite direction from town) is an orchid garden that is quite extensive. It used to belong to Raymond Burr the American actor. You could hire and a taxi and the driver will wait for you. the gardens are off the main road. They are worth a visit. I forget what the cost was exactly, but something like US$4 or so.
Don't forget your International departure tax, it used to be payable in Fiji Cash after check in just before your enter the departure lounge and duty free shop, though it may be included in some tickets. Be sure to ask if you need to pay it and how much it is.