We spent a week in Yasawas and a day or two in Manamucas.
I would recommend Fiji the best for divers, less for snorkeling, as the reefs are deeper.
We still enjoyed them a lot, but you needed to go quite deep comparing with Great Barrier Reef or Hawaii or Florida reefs.
Good surfing /windsurfing / beach cats - since you will be in a resort (we did a backpackers version
Nice kayak trip between islands, if it's not too windy. Some walking trails (most of the Yasawa or Mamanuca islands are too small to consider them "hiking trails").
Drink kava with the villagers.
If you like sailing, maybe visit on Spray would be nice: Seaspray Day Sailing Adventure - South Sea Cruises Fiji
, I was able to get some wheel time on it
It takes you to the island where "Castaway" movie was filmed.
One general negative about this place - we have been told, that during last few years the corruption in the government is getting worse, especially when connected with "Fiji water" company. So be mindful of your remarks in public. Friends are telling us that not a lot of changed since this was posted: Fiji Water: Spin the Bottle | Mother Jones