Wow there are just so many different day tours and cruises you can do now out to a stunning tropical island here if Fiji. If you're staying on the main land then it's a definite "must do" as you won't get the great snorkelling or beaches there.
Want more time on an island and less time in a boat getting you there, then take a South Sea Cruises trip to South Sea Island as its really close or PJ's to Bounty Island. The best snorkelling we've seen so far is at South Sea, Treasure, Octopus & Botaira - simply stunning. Treasure, Mana, Malolo, Castaway, Plantation, Octopus, Botaira & Robinson Crusoe Island have a resort where you spend the day exploring from. Here lunch options are very nice as they also cater to in-house guests. Other islands like South Sea, Bounty, Beachcomber, Tivua, Schooner & Savala are much smaller, lunches will be low key and generally BBQ buffet style - still yummy!
Only two islands have a jetty or wharf so no tendering onto a small boat there. The brand new jetty on Tivua Island and Mana Island has a big jetty.