Two week stop off in Fiji during round the world trip! What to do?

Hi, I will be arriving in Fiji on the 23rd of August for two weeks. Apart from using it as a nice chillout (as I will have just of come over from winter in New Zealand!) so snorkelling, sun bathing, drinking etc. I'm not sure what else there will be there? I have heard of a few shark dives that are offered around the island, any recommendations of some good ones? And also some under water caves you can visit. Also any recommendations of some younger hostels that still retain that lush resort kinda feel? The last thing is what island to visit!? I am completely lost with the amount of islands there are to visit...Thanks!

Whatever you do, minimize your time spent on the mainland.

Get the Bula Pass on the Yasawa Flyer and visit a bunch of the islands. It's the popular thing to do, with all sorts of resorts in the Yasawas and Mamanucas for all budget types, and plenty of young backpackers. 2-3 nights per resort is usually a good length of time before moving on. Blue Lagoon on Nacula island was my favourite, and from there you can take a short trip to the Sawa i Lau Caves (only goes on certain days).