Find an airline that allows small pets to be taken in the cabin. The cage has to fit under the seat but for a cat that shouldn't be an issue. It's much better than having them in the cargo. Once you are in the air you usually can take the cage in your lap which can help calming the cat down. But obviously taking the cat out isn't allowed.
Take the cat on drives as before the flight so it gets used to being in the cage and shaken around. It will also give you a rough idea how it responds to traveling.
A trip to the vet before the flight to get some sedative medication for the flight can also make the thing more comfortable for the cat.
Since the flight is long (according to a quick Google) considered intentionally taking a longer stopover midway. But since Hawaii is an island I don't know if there is anything in between to stopover. If the second flight is only 1-2 hours then a direct flight would obviously be better. Cats don't do well on an empty stomach but flying on a full stomach also isn't ideal. Having a prolonged stopover where you can feed them and then leave some time to digest would be more comfortable. If you got them used to walking on a leash you even can let them walk around the airport.
Other than that just bring wipes and plastic bags. Much easier to clean up any puke or other mess.