Andrea, if you have time on Maui, you should drop by Mama's Fish House in Paia for lunch. I think it's probably our favorite restaurant on Maui. Not cheap but we've never had a bad meal there.
There's a mom and pop place on Kauai called Hamura's Saimin Stand. Saimin is the local version of Japanese ramen (noodles in soup). I guess it would be our version of a diner...about 20 stools scattered around a counter.
My brain's too tired but I know there are MANY other places that we could recommend. I'll think about it some more and I'm sure Masaki will chime in later.