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.