Almost across the board, early- to mid-fall is a great time to visit national parks. Fewer visitors mean more room to enjoy the nature, the history, and the sights. And, the slightly cooler weather makes for more comfortable outdoor adventures.
This holds true for the eight national parks in Hawaii. The National Park Service has a handy map and links to its parks, which include Hawaii Volcanoes National Park (lava!) and the USS Arizona Memorial. In addition to the eight, other state and regional parks offer plenty more ways to get off the beach (at least for a few hours) and see the best and most beautiful of the islands.