On Maui and Kona, best bet is to rent a car. Public transportation is almost nonexistent except for an occasional taxi. Driving on the Islands is extemely easy, almost(notice I said almost) impossible to get lost. Confused perhaps but not lost. Even on the big Island, just head north or south and eventually you will come back to where you started. On Maui you can't go the whole way around the island(without violating your rental agreement) so eventually you will have to turn around. Car rental agencies give you these neat maps, very easy to follow, make great souveneirs, with lots of coupons to save you money if you spend money.
Dan