Fiji - a 12-hour layover

My girlfriend and I have a 12 hour layover (11:30am - 10:50pm) in Nadi on our way back to the states from Oz. We're young, outdoorsey, active, and into snorkelling and birding. Any good tips on what we could possibly see/do for an afternoon/evening on the island? Obviously, we're not going to see much, but a chance to get out of the airport and jump in the water or check out some birds up in the forest would be a welcome respit before our epic flight back to the States.

Thanks!