I've been to Hawaii at least twice a year for the last five years. My daughter lives on Oahu so that's where we've spent most of our time. There is so much to do on Oahu. I won't list the obvious don't misses, but, one place many seem to not know about is Kaena point. We hike to it from the north side. In March, and probably still in April, there are typically courting and nesting albatross. AMAZING.
We've been to Maui, Kauai and the Big Island. We're going back to Kauai for five days later this month. The Big Island is our favorite, other than Oahu. We are quite active and do a lot of snorkeling and hiking. Hilo and Kona on the Big Island feel more old time Hawaiian than other places. North of Kona on the Big Island tends to be more touristy. I would say the Big Island is much less expensive. Favorite snorkeling spots on the Big Island where we can access from shore are Carlsmith Beach in Hilo, Two Step and Kahalu’u Beach Park, both near Kona.
We found the traffic and construction on Kauai quite frustrating.
We enjoy learning about the culture and history of Hawaii. We have been to most if not all of the historic sites. The Big Island has wonderful historic sites. Also, Volcanoes park is quite special in terms learning and seeing the volcano and seeing some of the unique features. If you are into the night sky and gorgeous views, the summit of Mauna Kea is amazing.
I think people look for different qualities in terms of beach preferences. I think Carlsmith is one of the most beautiful beaches in Hawaii. It's definitely rocky, I guess some might not even consider it a beach, but gorgeous with decent snorkeling, always turtles. Some people like waves, some don't, some want pure sand, which is fabulous on the feet, but not typically great for snorkeling. The east side of Oahu has lovely sandy beaches, but it can rain more over there. A beach that is small, pretty and fun is the "From Here to Eternity" beach near the blow hole.