Yes the Big Island has lots of open roads. There used to be a group that would ship their bikes from Honolulu to the Big Island every few years for a few days ride. From what I hear, old Saddle Road was a popular WOT stretch (though I'm not into that).
Soon we'll have the Superferry (interisland ferry) so we won't have to cargo ship the bikes.
I've been up to Waimea Canyon on Kauai numerous times in a minivan (rental), and have always wished I was on a bike.
I do get worried up there though, as cell phone coverage is spotty. And if I ever broke down or crashed, I'd be waiting forever for help. Not to mention, for the life of me, I couldn't find any gas stations anywhere near the start of the canyon road.
On Maui, the road to the top of Haleakala is almost frustratingly beautiful. It's excruciating driving up there in a van wishing you had a bike. Nevertheless it's still a lovely journey.
Some pics from Maui (a few years old):
1) Brief view of the road leading up to the lookout.
2) View from one of the lookouts.
3) This is me, elevation ~10k feet. uuhhh this is what can happen to you when you don't obey safety signs.
