Haleakala on Maui can entail hiking 20+ miles and 10K elevation gain,wow!, from its northern slope. Beach to summit. Probably better to explore the crater near the summit.
There is some easy hiking at Iao Valley. But that doesn't sound like what you are looking for. The Iao needle is the big tourist draw, although unclimbable??
Kauai probably will provide you with the most opportunities. There are once in a lifetime views associated with Waimea Canyon (3600' canyon) and the beautiful Na Pali Coast. Check out the Kalalau Trail. Both of these areas are visually stunning.
I can also recommend Na Pali Coast and the Canyon. Years ago that I was there (still a kid) but the views and impressions where stunning. Though on Na Pali you probably need to walk back the way you came as the trail takes more than one day? Not sure.
Awa'awapuhi and Nualolo Trails form a loop starting (and ending) in Kokee State Park. I think it's about 11 or 12 miles, not alot of elevation gain, but boy, some of the most fantasic views I ever seen. You essentially hike out to the end of a ridge overlooking the Napali coast. There are some steep sections on the return leg, and some areas are washed out and exposed (at least as of June 04 when I was there).