Below are a few ideas but not at all comprehensive.
Two Jewels are Hawaiʻi’ National Parks – Hawaiʻi Volcanoes and Haleakala and their websites can be easily found with a web search.
National Wildlife Refuges
Hawaiʻi Island: https://www.fws.gov/refuge/hakalau_forest/
Molokai (Maui NWR complex site): https://www.fws.gov/refuge/kakahaia/
See the NARs page on this website.
A particularly interesting park is the Alakai Swamp (Kauai) which has interesting habitats and many endemic species: https://en.wikipedia.org/wiki/Alakai_Wilderness_Preserve.
Kokee state park (Kauai) is very nice for plants especially: http://www.kokee.org/index.php?kokee-flora-fauna.
Public places to see natural communities:
Nature Conservancy reserves or places they work: