Waimea
The "Grand Canyon of the Pacific" — 14 miles long, 1 mile wide, 3,600 feet deep. Deep red volcanic soil against tropical green and cloud wisps is genuinely awe-inspiring. Kalalau Valley Lookout at the end of the road is the only view into the roadless Na Pali interior. For a hike: from Pu'u Hinahina Lookout, take the Waimea Canyon Trail (also called "New Spur Trail" on Google Maps) to Waipo'o Falls (~5km round trip, 2–2h30) — completely uncrowded, spectacular canyon views throughout, and a beautiful falls at the end. Visit before 11am for the clearest lookout visibility; valley clouds build by midday most days.
No reservation required. Drive Kokee Road from Waimea town. $5/person state park entry. Stop at multiple canyon overlooks and end at Puu o Kila Lookout (4,000ft).
Visit before 11am for the best lookout visibility — valley clouds frequently obscure the view by midday.