For a 10 hour layover, don't risk stressing yourself out by trying to head out to one of the outer islands or over to Suva. Either of these would cost you a pretty $$, take a couple hours to go, a couple of hours to get back, plus an hour or so to check back into the airport etc. That doesn't leave you much time to really enjoy yourself.

Not really that much to see in Nadi so I'd agree with other posters and I'd take a cab out to Denarau Island and hang out on the beach / resort pool. If you don't want to just mooch free usage of the resort's pool facilities, I'd just find the cheapest room rate possible as all the resorts are on the beach. You can leisurely walk from the resorts at one end of Denarau to the resorts on the other end (maybe about 10 resorts?).

The Westin is really nice and just recently remodeled (last couple of years ago or so?). You can book a room at any of the three SPG properties (Sheraton, Sheraton Vilas or the Westin) and use any/all of the facilities. I'd try and book a room at the Sheraton as this is usually the cheapest.