Matched to fitness level, honestly
Easy
Diamond Head (Oʻahu) — 1.6 miles round trip, paved and stepped most of the way, big payoff view. Reservation required.
Waimea Canyon overlooks (Kauaʻi) — no hiking required at all; several major viewpoints are roadside.
Moderate
Kīlauea Iki Trail (Big Island) — about 4 miles through rainforest and directly across a solidified lava lake floor, inside Hawaiʻi Volcanoes National Park.
Pipiwai Trail (Maui) — a roughly 4-mile round trip past a bamboo forest to the 400-foot Waimoku Falls, near the end of the Road to Hana.
Strenuous
Kalalau Trail, first 2 miles to Hanakāpīʻai Beach (Kauaʻi) — a genuinely demanding, exposed cliffside trail even for the shortened version most day-hikers attempt; a permit is required to go beyond Hanakāpīʻai, and the full 11-mile trail to Kalalau Valley is for experienced backpackers only.
Haleakalā Sliding Sands (Maui) — descending into the volcanic crater is spectacular and the climb back out, at high altitude, is much harder than it looks on a map.
Standing near an active volcano, responsibly
Kīlauea, inside Hawaiʻi Volcanoes National Park on the Big Island, is one of the most active volcanoes on Earth and one of the very few where visitors can sometimes safely view active lava activity from marked, ranger-monitored overlooks. Conditions change fast — always check the current alert level and any area closures on the National Park Service conditions page and the USGS Hawaiian Volcano Observatory before you go, and never cross a closure line for a photo — volcanic gas (vog) and unstable ground are real hazards, not just rules.
Altitude is a real factor
Both Haleakalā (10,023 ft) and Mauna Kea (13,803 ft) summits are high enough to cause genuine altitude effects — shortness of breath, headache, dehydration — especially if you're driving up from sea level the same day. Go easy, hydrate well beforehand, and treat any dizziness or nausea seriously.