Oh man, I love the Hawaiian islands so much. We, too, did a trip which was supposed to be the only time. Huh. Loved it so much, we then saved up and went again 5 years later.
Agree with everything already said plus:
if you self drive, these guys provide awesome guided commentary (the road to Hana itinerary is linked below but there are others):
https://www.shakaguide.com/article/maui/shaka-guides-classic-road-to-hana-itinerary
On Big Island, please do not miss the diving/snorkelling with the mantas. It is EPIC
If in season, whale watching on Maui.
Eating/drinking can be very expensive, our local friends go to the happy hours instead and eat there which worked well for us. We also self catered a lot
I loved Kuaui, such a laid back, chilled out, stunning beautiful island. In Poipo, I got up to watch the sunrise and a seal came sliding out of the sea, dragged himself up the beach and went to sleep in the sand just a cars length away from me. Magical.
My least favourite island was Oahu. Didn’t love Waikiki but the north of the island was fab and, ofcourse, Pearl Harbour is a must do.