Having been to Oahu once, 30+ years ago (with a 24 hour side jaunt to the Big Island): There wasn’t much memorable in the Waikiki area itself. I mean, it’s lovely there, of course, and very different frmo home, but not anything extraordinary. You’ll need to venture a bit farther to find anything really interesting. You’ll want pool shoes of some sort if you swim in the ocean as the beach was a bit more rocky than an East Coaster like me is used to.
We were staying in a lower-priced hotel a block or so from the beach, as my husband was there for a conference at the university. I would drop him off every morning, then go do sightseeing things.
Not too far away, the most memorable things were Hanauma Bay, where you could snorkel in fairly shallow water with all sorts of tropical fish. Siam Sam can pop in with whether that’s still a worthwhile place to visit; it was very crowded when we were there, and parking was a big issue (it’s where we learned to follow people in the parking lot to see whether they were heading to a car, and thus vacating a space). My husband found out that it’s hard to shout when using a snorkel - somehow he was not expecting to have FISH swim right up to him, and it startled him. “Whaaaaaablubblubblub…:”
Sea Life Park was nice also - his conference group had an event there.
One day while he was at meetings, I drove around the eastern edge of island to the north shore, which was interesting. Not swimmable, IIRC - though there were surfers there. But driving back through the middle, through pineapple fields was pretty cool. I even ordered some pineapples to take home - they had a deal where you’re pre-order them, and pick them up at the airport.
Far more memorable was the Big Island. You’ll find that rental car places will say “cannot take xxx road” (some less-paved roads), so be careful of any restrictions. We stayed in Hilo, rather than Kailua Kona (I gather a more popular tourist destination?) because we wanted to see the volcanoes. So we went from tropical paradise, to cool, cloudy rain forest (up into Hawaii Volcanoes NP) , then down to the coast south of the lava flow which was like being in hell, in a good way: everything we drove past was BLACK, and there was a smell of sulfur, and it was overcast; the only color was when we parked,. and walked up as close as we safely could to where the active flow was happening, and we could see cracks in the lava where it was still glowing red-hot underneath. Oh, and the ocean was blue there too. Even the beach was black - because it was just crumbled lava. If you do that, LISTEN TO THE RANGERS - we were chatting with one who said he frequently had to chase peopel back off the solid-looking lava, as frequently it was just a cooled shell on top of actively HOT stuff.
I think we also stopped at a macadamia farm / processing plant and ordered macadamias to be shipped home, as well, but the volcanoes and lava flows were the most memorable things about the whole trip.
I seem to recall that the airfare, hotel and car rental for that 24 hour jaunt wasn’t too spendy - 300 bucks, maybe, for 2 of us (bear in mind, this was 1989, and it was just the flight from Oahu to Hilo).