5 days in Fiji in December

Hi All,
I fly into Nadi on Thursday, Dec 9th and fly out on Tuesday, Dec 14th. On a backpacker(ish) budget but not looking for party scene. I do like beaches but this will likely be my only 4 days ever in Fiji so would rather see the culture, natural wonders, get to know a but about the people/country. Any suggestions on key things to do at this time of year?, with only this amount of time?

Looking for recommendations for where to stay in Nadi the first night...there seems to be a lot of hostels.

Also I hear about a night hike up a volcano - any one know about this?

Thanks!

How about a homestay?

http://www.fijibure.com/

No volcanoes in Fiji though.