The Kalalau Trail Is Hands Down The Most Incredible Hike In America August 2018 Update: Nāpali Coast State Wilderness Park (including the Kalalau Trail) on Kauai are still closed due to flood damage as well as the highway and bridges which access the parks. sorry
I didn't get there, but my dad swears by the Waimea canyon hike in Kauai. It might be a good alternative if you can't get on Kalalau, which is as good or better than you imagine it to be. There's also a hike (can't remember the trail name) up to the waterfall on the big mountain that is visible from the north side of the island. On Maui I would recommend the Hoapili trail in Makena. It's a hike over a lava field, and one of the few places on the island where there aren't very many people, because (a) it's a bunch of sharp rocks, and (b) it's a pain in the ass to get to. Other than that any of the side trails I've been on around Hana highway are worth exploring.