I thought Big Island was fine but it's big and has lots of driving, and, other than Hilo, the landscape (lots of black lava rock) wasn't as interesting after about 10 minutes. To me the best spots were also in private resorts, and I'm cheap and don't like paying to get in. I'm sure you'd have fun but I'd start with Oahu and take its North Shore with Haleiwa and Matsumoto's Shave Ice, Pearl Harbor, Polynesian Cultural Center and (if LDS) BYU Hawaii and temple off the bucket list first. Also better snorkeling at Hanauma Bay and Shark's Cove on Oahu (though others would say Two Step and other places on Big Island are comparable). But if those things aren't compelling to you, or you just have to see a live volcano or snorkel with Manta Rays, then OK, do Big Island.

I say all this but also have to admit Kauai is my favorite island. Oahu is no. 2.