Fiji is certainly a lovely place and we enjoyed it greatly when we had a weeks mini holiday there, while touring NZ a few years back.
We stayed at the Shangri-La Fijian Resort which was lovely if not exactly cheap.
If you didn't want to travel that far, then could I recommend Malaysian Borneo.
Sarawak has lots of jungle walks and some lovely coastal scenery and Kuching is an interesting city (The City of Cats).
Sabah has the challenge of trekkiing up Mount Kinabalu, the famous Orangutan Sanctuary at Sepilok & offers a few decent top end beach resorts in the Kota Kinabalu area.
When we went, we had a week in KL/Penang on the mainland and then about 8 days in Borneo, which wasn't really enough but was certainly memorable.