The Hilton on Denarau is a fine place to stay. Most of the daily cruises leave from Denarau, so it is defintely convenient. There are plenty of F&B option on Denarau also.
There are plenty of cruises that include snorkeliing but you don't have to do any, if you don't want to. A nice one is "The Whale's Tale". A sail boat that goes to a couple of different islands.
Another different day trip is to Robinson Crusoe Island. Or up Nabua River. The Orchid gardens at the Sleeping Giant is a good 1/2 day trip. The biggest tour operator in Fiji is Rosie Holidays.
The othe Hilton property is Sonasali Island. Don't bother. It is quite far from Nadi and once you are on the island you are kind of stuck. The F&B is very hit and miss.