The Big Island is personally my favorite.
Go on a coffee plantation tour
Take a helicopter ride over the volcano
Go to the volcano park
Visit a cattle ranch
Snorkeling (we did a thing where you get on a dingy and go to a cove but there are dozens of places you can go right off the beach)
Take waterfall tours
Mauna Kea Observatory (this was my favorite, followed by the volcano)
Take a boat tour to visit all the caves the ocean has carved
See the "coral" graffiti on the black lava
Go to black sand beaches
We stayed at Waikoloa Beach Resort - my parents go every year and are there right now to come back next Friday. We went in August but I think now is when you can see the whales.
I haven't been to the Big Island in years but I doubt this has changed - there are no private beaches. That means you can sit on the beach in front of the chairman of Toyota's house and Bill Gate's house
Been to Oahu (Waikiki and North Shore) and Maui and The Big Island is still my fav.
We flew into Oahu one trip, stayed a night, went to Hawaii for a week then stayed a night in Oahu again on the way back. Try and do that so at the minimum you can see Pearl Harbor.
My brother and SIL also liked Kauai - they went hiking a lot. It's really easy and relatively cheap to fly from island to island. The only time I've taken a boat from one to another was Maui to Lanai.