Looks like some are also saying Kona. I'm not going to say anything against all of those - they're awesome.

That said, if you're a newbie and not as sure about all the things to do, I'd opt to (for now) start with the slightly more curated experience.

If you go stay in Honolulu, you can visit the PCC, Pearl Harbor, various great hikes, make an excursion to the North Shore, snorkel in Hanauama Bay. They're the common Hawaii things, but they're popular for a reason and you'll have a great time.

If you've done all that, then yes, absolutely, go a bit more off the beaten path the next time. But from where you're coming from, I'd say Oahu

I will edit to add - if you can set something up that allows you to travel between islands, that would be great for a first time. Base in Oahu and then go visit Kauai or Maui overnight and spend a day there and the variety of doing that will be amazing and whet your appetite to come back for more of that.