Here, listed geographcially from north to south in the Islands, are four of the best:
1. KILAUEA POINT NATIONAL WILDLIFE REFUGE, Kauai. On the north shore of the northernmost island is a stubby pennisula that is home to a who's-who of marine life—land, sea, and air. Very dramatic setting, free binoculars for the long views, and admission is a few bucks. The historic Kilauea Lighthouse, a classic white cylincer, adds romance to the place. On the same day you may see whales, spinner dophin, monk seals, Hawaiian nene (state bird), laysan albatross, and a variety of soaring shorebirds.
2. WAIKIKI AQUARIUM, Oahu. Not splashy (pun intended) in its presentation, but this University of Hawaii-run attraction has a beautiful setting—right below Diamond Head, opposite huge Kapiolani Park, and beside coral reefs just a short walk from Waikiki. There's more going on than meets the eye, both in the aquariums themselves and the special programs offered to visitors. Mid-range admission.
3. HUMPBACK WHALE NATIONAL MARINE SANCTUARY, Maui. This place just keeps getting better. The best time to visit, of course, is during winter migrations. It's on the underrated North Kihei Coast, next to a beach park that features an ancient fishpond re-creation and a mile from a birder's walk along a boardwalk. Admission is free.
4. HAWAII WILDLIFE CENTER, Big Island. This center welcomes visitors, though it is not a tourist attraction, but rather the only hospital in the Pacific Ocean that treats injured birds—shore, sea, and woodland. Funded through private donations and volunteer help, HWC opened just this year. An interpretive area for visitors is outside the attractive building. And the setting is green Kohala on the northern nub of the Big Island. If visiting the center, also check out 'Iole Foundation, a mile away, with its historic buildings and 2,400 acres of gardenscaped flora.
Trailblazer guides (Maui, Kauai, Oahu and the Big Island, No Worries Hawaii) have details on these places, as well as others that present the cutural and natural heritage of Hawaii.