What area of the Island are you staying? I'm more familiar with the North Shore and can give you some more info if you're staying there at all.
"Grand Canyon of the Pacific" is amazing.
North Shore is beautiful and the Napali Coast is must see. Hike it, take a boat, see it by helicopter. It's like an entire 1/3rd of the island with no roads that is smack you in the face beautiful. If you're into hiking, you can actually hike a good portion of the Napali Coast on the Kalalau Trail and camp on a beach that's only accessible by hiking or boat. I didn't make it as far as the beach when I went, but wish I had of. Looks unreal. You can also just day hike it and there's also a pretty nice waterfall/swimming hole detour (Hanakoa Falls) on that same trail that can be done same day in and out.
Kalalau Trail is often closed in winter though due to conditions.