I would recommend checking out: http://www.awesomefiji.com/
Me and a couple of friends went through Awesome Adventures Fiji and had a great time for what I thought was pretty cheap. They have all types of packages ranging from island jumping, stayputs, party cruise, all-inclusive etc. all of which are within Yasawa and Mamanuca islands. We stayed at Kuata Island and Beachcomber. Kuata is a very simple, laid back, and a beautiful island with extremely friendly natives that work there, Beachcomber is a small, nice, "party" island with lots of activities, and good food. I hope this helps, if you have any questions let me know I would be more than happy to help out. Good luck.