Bula! So 5 days, non diving partner, US$200 / night. I assume diving extra.
Time limit really rules out places that need an internal flight, although they are the best spots, so look at Viti Levu and Beqa Island. Options are Volivoli, Wananavu, Beqa Lagoon Resort, Waidroka, Uprising and the Pearl. Volivoli is a lovely but isolated resort with rooms at US$175 rack rate for a couple but they have great offers to include dives, so ask. Wananavu is a little more upmarket but as it is low season, they too have good offers. Book dives in advance as walk in rates are v high. Both places dive the same area which can be stunning. Non diving stuff somewhat limited outside of the resort.
If you like sharks, go to Pacific Harbour or Beqa Lagoon Resort. There are 3 shark feeding dives there. One run on Beqa island by Beqa Lagoon Resort, and the other 2 from Pacific Harbour, being Aquatrek and Beqa Adventure Divers.
They are similar and everyone has their preference, but all professionally run. In PAcific HArbour you can stay at Uprising and pay around US$140 inc breakfast with a pay 4 stay 5 option I think. Or stay at the Pearl but that would probably bust the budget a little. If you do, book Garden View rooms not the Marina view - trust me....
Beqa lagoon resort is great too. The only down side is you need a boat transfer and there may be a surcharge if not on a scheduled day. Rates for 5 night at under US$1,000 for 2. I'm not sure if that includes a meal plan.
If I was to pick for you, I would stay at Uprising, dive with Aquatrek, booked in advance, doing at least 1 days shark dive. Staying in PH gives you access to loads of activities modern and traditional.
Food for though. Which makes me realise it's time for breakfast.
Greg