You will definitely need a car in both Kauai & Maui.

As for information, a good place to start is the different visitors bureau.

Kauaivisitorsbureau.org
visitmaui.com

Poipu Point is a good part of Kauai to stay, but defintely make an effort to drive up to the Princeville area too, it's well worth the drive.

Another site I found helpful is the 101 things to do in Hawaii website - 101things.com They have information for all HI island.

Hope this helps a little. Have fun & ALOHA!