Tour Operators in Viti Levu

I am reading the LP guide on Fiji and have seen several adventure tour operators listed there, but I'd like to get word of mouth recommendation from other fellow TT travelers (word of mouth recommendations have worked perfectly in the past for me)

I am looking to do mountain biking or hiking through stunning scenery in the highlands and spend one night in a remote fijian village and see how the people live...I was also interested in Whitewater rafting, but i'm not sure of the quality of Fiji's rapids (would prefer class III to V rapids)
Which reputable tour agencies would you recommend based on past experiences?

I was also looking to go to the Yasawa islands for 2 days and was looking to do an introductory scuba dive since I am not certified, in addition, I'd like to see and experience some stunning beach and sunset scenery, play some sports like volley and do some snorkeling and jet ski.....which of the Yasawa islands should I visit for this?

One more question: where should I stay in the mainland for 3 to 4 nights? Nadi or Lautoka or the Coral Coast? I am a solo travelers, so need to watch my hotel budget...

Any advise on these is highly appreciated!