Why these two aren't in the main six
Hawaiʻi has eight main islands total, but this site — like most practical travel planning — focuses on the six that are actually visitable: Oʻahu, Maui, Hawaiʻi Island, Kauaʻi, Molokaʻi and Lānaʻi. Niʻihau and Kahoʻolawe are both real, and both genuinely off the table for a typical visit, for very different reasons — worth understanding rather than just ignoring.
Niʻihau, "the Forbidden Isle"
Privately owned by the Robinson family since 1864, Niʻihau has been kept deliberately isolated from outside development and tourism for generations, specifically to preserve a small Native Hawaiian community living a traditional, largely self-sufficient lifestyle — it remains the last place in the state where ʻŌlelo Hawaiʻi (Hawaiian) is the first language of daily life for most residents, rather than a second or ceremonial language. General visitors cannot go — there is no public ferry, no hotel, no open invitation. A small number of tightly limited helicopter tours and hunting safaris have operated at times, landing on the island but not entering the village, though availability changes and should be checked directly rather than assumed.
Niʻihau shell lei — made from tiny, genuinely rare shells found only on the island's beaches — are a distinct, highly valued craft tradition, and authentic pieces (sold on other islands, since Niʻihau itself isn't accessible for shopping) can be genuinely expensive collector's items, reflecting real scarcity and skill rather than tourist markup.
Kahoʻolawe, the island being restored
The smallest of the eight main islands, Kahoʻolawe has no permanent population and, unlike Niʻihau, is not privately held — it's held in trust by the State of Hawaiʻi. The island's modern history is difficult: used as a US military bombing range from World War II through 1990, which caused severe environmental damage and left unexploded ordnance across much of the island. A sustained Native Hawaiian-led protest movement in the 1970s–80s led to the bombing's end and the island's transfer back to the state, and restoration and cultural work — led by the Protect Kahoʻolawe ʻOhana and the Kahoʻolawe Island Reserve Commission — has continued since.
Access is essentially limited to organized cultural and restoration work trips, not tourism — there's no general public visitation. If the island's history and ongoing restoration interest you, the Protect Kahoʻolawe ʻOhana and the state reserve commission are the right places to look for accurate, current information, rather than a travel guide.