10 days, just Yasawas or Yasawas/Viti Levu?

Next October will spend 10 days in Fiji (single Backpacker), planning to stay 4 nights in Blue Lagoon Beach (dorm) Resort but have to decide what to do for next days, choose other place in the Yasawas (octopus resort?) or spend some days in viti levu- Beachouse to see another island?

Thanks in advance for any reply!!!

Giuseppe
http://www.appuntidiviaggi.net (Italiano/English)