When we took, the POA, we rented a car in a couple ports in advance through Discount Hawaii Car Rentals. https://www.discounthawaiicarrental.com/en-us/. In Kaui, where we had an overnight, we rented the car for the day, downloaded, the gypsy app for Waimea Canyon and Kokee State Park. it was great. Then, the guys played a little golf and we had an early dinner in Poipu Beach and returned the car. The next day in Kauai, we took a shuttle to the beach.
In Maui, we took the NCL excursion- Halealakala at Sunset and my DH and son loved their NCL snorkeling excursion in Kona. Both were great.