good luck on this one.. its hard to get a car there for everyone.. if you’re under 21 its nearly impossible… try “ugly duckling rentals”.. they used to be on four islands but i dont know if they still exsit there..
you can rent scooters and motorcycles though.. alot easier but not as convienent.. aloha

you real question should be are you willing to pay the additional $25/day underage fee?

or look for codes-even expired ones- then argue with the manager till you get your way…. i’ve actually done this with hertz twice, and i got the code from studentadvantage
aloha! e komo mai to hawai’i ne!

Try this website:
Someone else recommended it on YA a few months ago & we used it when my 23 year old son went to Kauai.

Not sure why that didn’t link, but you can cut & paste it into your browser.

PS, when we went through this web site, we were able to avoid the $25 surcharge.