another vote for Big Island.
Really none of the islands are 'bad', we're basically splitting hairs.
For a first timer doing Oahu and seeing Waikiki beach is something to see + there is Pearl Harbour there another thing to see. And there is Hanauma Bay for snorkeling which is incredible. North Shore Oahu to see surfers and surf competitions, big food truck scene, but you won't be snorkeling or swimming here....
But otherwise we like the Big Island. Kona side. LOTS of great beaches, just wade out and snorkel and see a million fish. Snorkeling near Captain Cook Monument is ideal. Although I will say that when we went in August it was HOT but much nicer water for snorkeling. January has more waves thus more silt is churned up, making the visibility less. In August it was like 30m, just incredible.
Big Island is also cheaper than other islands IMO. Still expensive mind you, but cheaper. And you can usually get a direct flight.
I found Kauai too rural. A little TOO slow for me. Oahu WAY too fast paced/busy city for me. Big Island is a nice mixture and right pace.
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