The only indigenous Hawaiian land mammal is a bat, so plants here didn't evolve to yield much food for humans. Besides the Vaccinium and Rubus mentioned above, I've been curious to try lama (Diospyros hillebrandii), a persimmon relative. My notes read "Sweet but scant fruit pulp. Maybe fun to select for more pulp?"
Mamaki (Pipturus albidus) has edible inoffensive fruit, but I wouldn't go out of my way to eat it.