Like most other Tuggers, I start with a reservation, (ANY reservation) for the dates and car I need on the island I'm visiting. After I have that starting point reservation, I register it with Autoslash.com. They will start there, and search for you to find a lower rate. I've found they often can find things cheaper than I can find for myself. If they find a lower rate, I reserve that vehicle, cancel the first reservation, and keep moving ahead. I've had a number of times where the autoslash rate continues to drop, so I repeat the reservation process to get the newer, lower rate, and cancel the previous reservation. By the end of the time frame, I've ended up with very good rates with a major company. No surprises, and all is well.
But I also keep checking Costco. I've found they sometimes have sales, or days when rates are lower than expected, without me having to do anything special. I've never had Autoslash tell me they found a lower rate, and have it be with Costco. It's always been with one of the other companies. So keep pinging the Costco Travel car rental site. All the rates I quoted above for my existing reservations are booked through Costco. So far, Autoslash hasn't been able to beat those numbers, even though all three reservations are registered with them.
As to the "best" car rental agency on Oahu? I think they're all about the same. Stick with the brand names you know (Alamo, Budget, Hertz, etc.) and chances are you'll be fine. I'd avoid any sort of "Rent a Wreck" company, because chances are good those cars are truly bad.
Congratulations to your Granddaughter. My Dad was a career mustang Coast Guard officer, (Enlisted --> CPO --> Warrant Officer --> LDO), and I know how difficult it can be to get promoted.
Dave