Indeed you are sooo... unfortunate to be going to two of my favorite HI islands- Kauai and the Big Island!

Pgupta011 gives good advice here as always. I'd add that you can also check with hotels for combo's with a rental included. I once last year took a car with Budget (on the Big Island) and the Hilton Waikoloa Village for example at an excellent price, which was even in the Towers section with an Ocean view.

Also consider that even a straight leisure rental with typical AAA discounts are not that expensive in HI on a relative basis! Just plan ahead and shop for the best rates.

I'll keep my eyes peeled for cert's that are good and certainly post what I find if anything! Have a great trip!