Yesterday evening we booked our interisland flights with Priceline.com. We booked flights between 8:30am and 10:00am so we can leave one island and still enjoy at least half a day on the other one.
For our rental cars we choosed Hertz. We know there are cheaper rental companies but they have clean and new cars :-) We also choosed German contracts because for us it’s more convenient with German contracts instead of English ones (i.e. via Priceline.com) in regard to taxes and assurances (i.e. CDW).