Fiji is a beautiful place but often only used as a 2 or 3 day stopover on the way from Europe to New Zealand and Australia.  Give yourself weeks not days (months if you have the time) and explore it.  Get away from Nadi, explore islands other than Viti Levu and most of all get away from the big tourist resorts and see the real Fiji.  There are plenty of web sites  about Fiji. Also consider learning to dive at reasonable prices in excellent conditions.

If you want to explore the rest of the South Pacific and Micronesia, Fiji is a pretty good base with regular flights to Kiribati, Marshall Islands, Guam, Solomon Islands, Vanuatu, New Caledonia, Tahiti, Tonga, Western Samoa, Cook Islands, Nauru and others.