Kaui was our last trip in 2020 before the world changed. Natural beauty is incredible: Waimea Canyon and Napali Coast are geographic bewilderments that can only be appreciated in person. A few top recomendations:

Waimea Canyon: you can drive to several lookouts within the park (and you will need a car on Kauai). When we were there, there were a few native Hawaiians doing an unofficial presentation about culture and history on the island that was very interesting and authentic.
Napali Coast: Cannot be seen or accessed from a road. The hike is one of the best on Earth, but professional grade difficult. You can see it by helicopter. Best seen by water IMHO: there are many catamarans that go out for day cruises, I reccomend sunset, but a day cruise will include snorkeling. Can also be accessed by kayak tour, and there are beaches that can be camped.
rivers and waterfalls This is the only Hawiian island with navigable rivers. Lots of waterfalls throughout the island. Secret Falls kayak/hike checks lots of boxes in one trip and easy to rent a kayak. Take tandems, its a harder paddle back than it is up, but kids can make the trip. It is crowded. More secluded adventures for more intrepid travelers.

Here's my full boating report with a few photos Busy Boater: Kaui

We've also been to Oahu/Waikiki a few times. Special place in my heart because it was my first vacation with my wife. My favorites: Diamond Head Hike, Waikiki Beach Catamarans, and wandering through the midcentury hotels. We rented a car the first time we were in Oahu and never pulled it out of the garage. Driving around Waikiki is a PITA and it is very walkable. But if you are only there for 1-2 days and going to Arizona, it might make sense.

Diamond Head: Volcanic Crater with WWII military installation and beautiful views. You can drive into the crater and hike to the top. Its crowded, the trail is pretty improved, but lot of elevation change. I've walked from Waikiki to the top of the crater and back twice in one day, and I'm an average walker.
Beach Catamarans If you have some free time, walk up to any of the catamarans on the beach and they will have a cash price for 1-hour sale. Cheaper than online. Years ago I think we paid $6 for the sail, $1 for mai tais and $2 for Budweiser. Stick with the Budweiser. Beautiful to see Waikiki from the water.
Hotels So many historic hotels. Not all in great condition, but nonetheless, memories from bygone eras. Sheraton from Brady Bunch. Surf Rider from Blue Hawaii. Kaimaina Beach Hotel has a really cool old bar/lobby that still looks as it did on Magnumm PI. Hyatt is designed like a John Portman atrium hotel, but there's no roof on the atrium! Royal Hawaiian was a bust last time we were there (2019).
BeachThe beach is great for boogie boarding or learning to surf too. For more peace and quiet, head south through the park to Kamaina Beach.

Here's my Oahu boating report Busy Boater: Oahu

Feel free to DM me with other questions and have a great trip!