Fiji help

My significant other and I, have decided to go to Fiji for our 20th anniversary next year. Which means it’s time to start planning… how do you decide which island(s) to go to? I assume that’s where we should start. We like hiking, rafting (exploring really) & relaxing on the beach (some white sand beaches),… would love to make the most of the trip obviously and really experience some culture too. We were thinking about eight days (not including travel days).

Would appreciate any starting suggestions!