You might start off with anything you think may be a "must see" or "must do" on each island.
Don't know how much time you will have each day on Oahu but for us Pearl Harbor was a must see. How much time you have between arriving on Oahu and boarding the ship will make a difference in what you can do. Same for when the cruise arrives back to Oahu and you have to be at the airport. Waikiki Trolley, a hop on/hop off trolley (there are others), if there is a pickup spot close to the cruise ship (I think the Aloha Tower is close) is a great way to get around the island. They have different color lines that have specific themed routes, such as cultural, scenic, historic. It is, IMO, the best way to get to Pearl Harbor. I didn't get my way in setting up this excursion and we wound up doing an excursion through Viator/TripAdvisor - it was not worth it at all. You can do it all on your own. We did the Diamond Head Scenic Tour and went through the zoo and aquarium. They had one monk seal when we were there. Oahu, of the 3 islands we've been on is worth doing the topside stuff.
Don't know anything about Maui, unfortunately, but 2 days there seems like it could give you an opportunity to get some dives in as well as sight see.
@diversteve gives you good info on that.
Hilo isn't the side of the Big Island to dive on and you probably don't want to try and cross the island to Kona, where you will be the next day, to dive as crossing back over the high elevation after diving wouldn't be good for you. But, a helicopter ride or trip to Volcano National Park (some parts are closed, but a lot is open) are some topside activities there.
Kona would be a great place to dive (I'm going back at the end of Oct.) if you can work it out. Sunnier and drier than the Hilo side. If you find that you don't have to be back on the ship until later at night, the Manta Ray snorkel/dive is one of those must-sees.
Again, 2 days in Kauai would seem to give you the opportunity to dive. I would check with Seasport Divers or Fathom Five which both dive in Poipu. If you can't get out on boat dives with them, they (Fathom Five for sure) will guide you on shore dives at Koloa Landing. If you do the shore dives, be sure to have dive boots as you walk out over larger rocks to get to a float before going under. I would guess that the time to be back on your cruise ship is a little later in the evening as a couple of the evening luaus are an excursion from the cruise ships - just an observation. Might be true on each of the islands.
We would love to dive every day while in Hawaii but after reading some threads about the diving not being all that great and the topside so amazing we decided to mix it up.
As
@KathyV described, diving in Hawaii is different from the Carribean. To the threads that say it's not being that great - I don't know where else I would have seen 100's of spinner dolphins swimming and a group of 5 or 6 breaking off and swimming right to me and my camera in Kona or a humpback whale while diving in Kauai. And of course, there are the mantas in Kona. Also saw a monk seal in Kona.
I'm doing only my second cruise in January to the Caribbean and plan to dive on a couple of islands. A consideration is if I'm taking my dive gear or renting. The only thing I know I'm taking for sure is my prescription mask.