There is no way I would go to Hawaii - any island - and not rent a car. Particularly on Maui & Kauai. The cost of the cabs alone will come close to renting a sub-compact.
The Hyatt in Maui is a good walk to Whaler's Village and a 15 minute drive to Lahaina. I haven't been to Kauai yet.
Even on Oahu I rent a car. There is so much more to the island than Waikiki, or even the circle bus ride, to see. While TheBus can take you anywhere, I like the freedom to come & go as I please.