Denerau is the wrong location for all of those activities
I stayed at the Westin Denerau and loved it, but Denerau is a man-made island (reclaimed land) with a completely artificial environment, so there is no nature walks (except on the golf course), cultural activities consist of nightly fire dances at the resort, and the resort food is all imported from Australia for the most part. Still you get the friendly Fijian staff and it is great ... but for the things you list, anywhere in Fiji outside of Denerau is better. There isn't even a decent beach at Denerau due to a current which washes away the sand and makes the water murky (probably the only part of Fiji that doesn't have a great beach).