I’m on my way to Oahu now. A few things: if you want to do the night Manta Ray dive, sign up very early. Gets sold out. Leaves from the island of Molokai, which must be accessed from a flight or ferry from Maui. Not the simplest destination.
Also, currently, there are very few rental cars in HI. Seems most companies sold their fleet during the pandemic. There’s very limited public transit, and mostly on Oahu. So check before booking your flight!