Hawai'i Volcanoes National Park. Steam vents, lava tubes and craters. Check the park's current conditions before you go, because what is active changes.
Manta ray night snorkel. Off the Kona coast, and one of the few genuinely unforgettable things you can do in Hawaii. Runs year round.
Mauna Kea at sunset. Above the clouds, and cold enough that people are caught out. Bring layers, and check the access rules before you drive up.
Black and green sand beaches. Punalu'u for black sand, and a hike out to Papakolea for green. Both are a commitment from the Kohala side.
Kona coffee farms. The only place in the United States that grows it commercially at this scale. Most farms take visitors.
Waipi'o Valley lookout. One of the best views on the island. Access down into the valley has been restricted, so plan on the lookout.